Phentermine 37.5mg Tablets
It is used for weight loss.
Drug Class
Anorexiant; Weight management
Pharmacologic Class
Sympathomimetic amine; CNS stimulant
Pregnancy Category
Category X
FDA Approved
May 1959
DEA Schedule
Schedule IV
Overview
What is this medicine?
Phentermine is a prescription medication used for short-term weight loss. It works by affecting certain chemicals in your brain to help reduce your appetite. It should be used along with a doctor-approved diet, exercise, and behavior change program.
How to Use This Medicine
Taking Your Medication Correctly
To get the most benefit from your medication, it's essential to take it exactly as directed by your doctor. Carefully read all the information provided with your medication and follow the instructions closely. The dosage schedule may vary depending on the specific product you're taking. Some products are designed to be taken once a day, either before breakfast or 1 to 2 hours after breakfast. Others may need to be taken multiple times a day, approximately 30 minutes before meals. Make sure you understand the correct dosage schedule for your medication.
To minimize potential sleep disturbances, avoid taking your medication late in the evening. If you have any questions or concerns about taking your medication, be sure to discuss them with your doctor or pharmacist.
Storing and Disposing of Your Medication
To maintain the quality and safety of your medication, store it at room temperature in a dry place, away from the bathroom. Keep your medication in a secure location where children cannot see or reach it, and where others cannot access it. Consider using a locked box or area to store your medication. Remember to keep all medications out of reach of pets.
When disposing of unused or expired medication, do not flush it down the toilet or pour it down the drain unless you've been instructed to do so. Instead, check with your pharmacist for guidance on the best disposal method. You may also want to explore local drug take-back programs in your area.
What to Do If You Miss a Dose
If you miss a dose, take it as soon as you remember. However, if it's close to the time for your next scheduled dose, skip the missed dose and resume your regular dosage schedule. Do not take two doses at the same time or take extra doses to make up for the missed one.
To get the most benefit from your medication, it's essential to take it exactly as directed by your doctor. Carefully read all the information provided with your medication and follow the instructions closely. The dosage schedule may vary depending on the specific product you're taking. Some products are designed to be taken once a day, either before breakfast or 1 to 2 hours after breakfast. Others may need to be taken multiple times a day, approximately 30 minutes before meals. Make sure you understand the correct dosage schedule for your medication.
To minimize potential sleep disturbances, avoid taking your medication late in the evening. If you have any questions or concerns about taking your medication, be sure to discuss them with your doctor or pharmacist.
Storing and Disposing of Your Medication
To maintain the quality and safety of your medication, store it at room temperature in a dry place, away from the bathroom. Keep your medication in a secure location where children cannot see or reach it, and where others cannot access it. Consider using a locked box or area to store your medication. Remember to keep all medications out of reach of pets.
When disposing of unused or expired medication, do not flush it down the toilet or pour it down the drain unless you've been instructed to do so. Instead, check with your pharmacist for guidance on the best disposal method. You may also want to explore local drug take-back programs in your area.
What to Do If You Miss a Dose
If you miss a dose, take it as soon as you remember. However, if it's close to the time for your next scheduled dose, skip the missed dose and resume your regular dosage schedule. Do not take two doses at the same time or take extra doses to make up for the missed one.
Lifestyle & Tips
- Take phentermine exactly as prescribed by your doctor, usually once daily in the morning before breakfast or 1-2 hours after breakfast.
- Do not take phentermine late in the day (e.g., after early afternoon) as it can cause insomnia.
- Do not take more than the prescribed dose or for longer than recommended, as it can be habit-forming.
- Follow a reduced-calorie diet and engage in regular physical activity as advised by your healthcare provider.
- Avoid alcohol while taking phentermine, as it can increase side effects.
- Inform your doctor about all other medications, supplements, and herbal products you are taking.
- Do not stop taking phentermine suddenly without consulting your doctor, especially if you have been taking it for a long time, as withdrawal symptoms may occur.
Available Forms & Alternatives
Dosing & Administration
Adult Dosing
Standard Dose:
37.5 mg orally once daily in the morning, before breakfast or 1-2 hours after breakfast.
Dose Range:
15 - 37.5 mg
Condition-Specific Dosing:
obesity:
For short-term (a few weeks) use in conjunction with a regimen of weight reduction based on caloric restriction, exercise, and behavior modification in patients with an initial BMI of 30 kg/m² or greater, or 27 kg/m² or greater with at least one weight-related comorbidity (e.g., hypertension, diabetes, dyslipidemia).
Pediatric Dosing
Neonatal:
Not established
Infant:
Not established
Child:
Not established
Adolescent:
Not recommended for children under 16 years of age due to lack of safety and efficacy data.
Dose Adjustments
Renal Impairment:
Mild:
No specific dose adjustment provided; use with caution.
Moderate:
Use with caution; consider lower doses (e.g., 15 mg) and monitor for adverse effects.
Severe:
Contraindicated due to potential for accumulation and increased risk of adverse effects.
Dialysis:
Not available; likely contraindicated due to severe renal impairment.
Hepatic Impairment:
Mild:
No specific dose adjustment provided; use with caution.
Moderate:
Use with caution; consider lower doses and monitor for adverse effects.
Severe:
Contraindicated due to potential for accumulation and increased risk of adverse effects.
Pharmacology
Mechanism of Action
Phentermine is a sympathomimetic amine with pharmacologic activity similar to amphetamines. It primarily acts as an indirect-acting adrenergic agent, stimulating the release of norepinephrine and dopamine from nerve terminals in the hypothalamus, particularly in the lateral hypothalamic feeding center. This leads to appetite suppression. It may also have direct effects on satiety centers in the brain.
Pharmacokinetics
Absorption:
Bioavailability:
Not precisely quantified, but well absorbed from the GI tract.
Tmax:
3-4.4 hours
FoodEffect:
Can be taken with or without food, but absorption may be slightly delayed with food.
Distribution:
Vd:
Not widely reported, but distributes well throughout the body.
ProteinBinding:
~17%
CnssPenetration:
Yes
Elimination:
HalfLife:
19-24 hours (highly dependent on urinary pH; acidic urine decreases half-life, alkaline urine increases it)
Clearance:
Primarily renal clearance
ExcretionRoute:
Renal (urine)
Unchanged:
70-80% (in acidic urine)
Pharmacodynamics
OnsetOfAction:
Within hours
PeakEffect:
3-4.4 hours
DurationOfAction:
12-14 hours (due to long half-life, allows for once-daily dosing)
Safety & Warnings
Side Effects
Urgent Side Effects: Seek Medical Help Right Away
Although rare, some people may experience severe and potentially life-threatening side effects while taking this medication. If you notice any of the following symptoms, contact your doctor or seek immediate medical attention:
Signs of an allergic reaction, such as:
+ Rash
+ Hives
+ Itching
+ Red, swollen, blistered, or peeling skin with or without fever
+ Wheezing
+ Tightness in the chest or throat
+ Trouble breathing, swallowing, or talking
+ Unusual hoarseness
+ Swelling of the mouth, face, lips, tongue, or throat
Signs of high blood pressure, including:
+ Severe headache or dizziness
+ Passing out or changes in eyesight
Changes in behavior or mood
Shakiness
Rare but serious side effects, such as heart valve problems or increased pressure in the lungs, which can be fatal. If you experience any of the following, call your doctor right away:
+ Dizziness or passing out
+ Persistent tiredness or weakness
+ Fast or abnormal heartbeat
+ Chest pain
+ Shortness of breath
+ Swelling in your arms or legs
+ Decreased ability to exercise
Other Possible Side Effects
Like all medications, this drug can cause side effects. While many people may not experience any side effects or only minor ones, it's essential to discuss any concerns with your doctor. If you experience any of the following side effects, or if they bother you or persist, contact your doctor:
Dizziness or headache
Feeling nervous or excitable
Diarrhea or constipation
Dry mouth
Trouble sleeping
Bad taste in your mouth
Decreased interest in sex
Difficulty getting or maintaining an erection
Restlessness
This is not an exhaustive list of possible side effects. If you have questions or concerns, consult your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
Although rare, some people may experience severe and potentially life-threatening side effects while taking this medication. If you notice any of the following symptoms, contact your doctor or seek immediate medical attention:
Signs of an allergic reaction, such as:
+ Rash
+ Hives
+ Itching
+ Red, swollen, blistered, or peeling skin with or without fever
+ Wheezing
+ Tightness in the chest or throat
+ Trouble breathing, swallowing, or talking
+ Unusual hoarseness
+ Swelling of the mouth, face, lips, tongue, or throat
Signs of high blood pressure, including:
+ Severe headache or dizziness
+ Passing out or changes in eyesight
Changes in behavior or mood
Shakiness
Rare but serious side effects, such as heart valve problems or increased pressure in the lungs, which can be fatal. If you experience any of the following, call your doctor right away:
+ Dizziness or passing out
+ Persistent tiredness or weakness
+ Fast or abnormal heartbeat
+ Chest pain
+ Shortness of breath
+ Swelling in your arms or legs
+ Decreased ability to exercise
Other Possible Side Effects
Like all medications, this drug can cause side effects. While many people may not experience any side effects or only minor ones, it's essential to discuss any concerns with your doctor. If you experience any of the following side effects, or if they bother you or persist, contact your doctor:
Dizziness or headache
Feeling nervous or excitable
Diarrhea or constipation
Dry mouth
Trouble sleeping
Bad taste in your mouth
Decreased interest in sex
Difficulty getting or maintaining an erection
Restlessness
This is not an exhaustive list of possible side effects. If you have questions or concerns, consult your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
Seek Immediate Medical Attention If You Experience:
- Chest pain
- Shortness of breath
- Fainting
- Swelling in your ankles or feet
- Severe headache
- Blurred vision
- Palpitations or a racing heart
- Unusual changes in mood or behavior (e.g., severe anxiety, agitation, hallucinations)
- Signs of an allergic reaction (rash, itching, swelling, severe dizziness, trouble breathing)
Before Using This Medicine
Before Taking This Medication: Important Information to Share with Your Doctor
It is essential to inform your doctor about the following conditions and circumstances to ensure safe treatment:
Any allergies you have, including allergies to this medication, its components, or other substances, such as foods or drugs. Describe the allergic reaction and its symptoms.
A history of heart disease, including heart failure or abnormal heart rhythms, as well as high blood pressure or stroke.
Presence of certain health conditions, including glaucoma, agitation, anxiety, or an overactive thyroid gland.
Kidney disease or being on dialysis, as this may affect how your body processes the medication.
Recent use (within the last 14 days) of specific medications for depression or Parkinson's disease, such as isocarboxazid, phenelzine, tranylcypromine, selegiline, or rasagiline, due to the risk of severely high blood pressure.
Current use of certain medications, including fluoxetine, fluvoxamine, linezolid, methylene blue, paroxetine, or sertraline, as these can interact with this drug.
Use of any other weight loss medications, whether prescription, over-the-counter, or natural products.
Pregnancy or the possibility of being pregnant. This medication should not be taken during pregnancy.
* Breast-feeding. You should not breast-feed while taking this medication.
This list is not exhaustive, and it is crucial to discuss all your medications (prescription, over-the-counter, natural products, and vitamins) and health conditions with your doctor and pharmacist. They will help determine if it is safe to take this medication with your other drugs and health problems. Never start, stop, or change the dose of any medication without first consulting your doctor.
It is essential to inform your doctor about the following conditions and circumstances to ensure safe treatment:
Any allergies you have, including allergies to this medication, its components, or other substances, such as foods or drugs. Describe the allergic reaction and its symptoms.
A history of heart disease, including heart failure or abnormal heart rhythms, as well as high blood pressure or stroke.
Presence of certain health conditions, including glaucoma, agitation, anxiety, or an overactive thyroid gland.
Kidney disease or being on dialysis, as this may affect how your body processes the medication.
Recent use (within the last 14 days) of specific medications for depression or Parkinson's disease, such as isocarboxazid, phenelzine, tranylcypromine, selegiline, or rasagiline, due to the risk of severely high blood pressure.
Current use of certain medications, including fluoxetine, fluvoxamine, linezolid, methylene blue, paroxetine, or sertraline, as these can interact with this drug.
Use of any other weight loss medications, whether prescription, over-the-counter, or natural products.
Pregnancy or the possibility of being pregnant. This medication should not be taken during pregnancy.
* Breast-feeding. You should not breast-feed while taking this medication.
This list is not exhaustive, and it is crucial to discuss all your medications (prescription, over-the-counter, natural products, and vitamins) and health conditions with your doctor and pharmacist. They will help determine if it is safe to take this medication with your other drugs and health problems. Never start, stop, or change the dose of any medication without first consulting your doctor.
Precautions & Cautions
It is essential to inform all your healthcare providers, including doctors, nurses, pharmacists, and dentists, that you are taking this medication.
When starting this medication, be cautious and avoid driving or engaging in activities that require your full attention until you understand how it affects you.
Adhere to your doctor's prescribed treatment duration and dosage instructions. Do not take this medication for longer than recommended, as prolonged use or high doses can lead to tolerance, reducing its effectiveness. If you experience a decrease in the medication's effectiveness, consult your doctor. Avoid taking more than the prescribed dose.
If you have been taking this medication for an extended period, consult your doctor before stopping treatment. Your doctor may recommend a gradual tapering off the medication to minimize potential withdrawal effects. Be aware that this medication can be habit-forming, and long-term use should be avoided. If you have a history of substance or alcohol abuse, inform your doctor.
Before initiating treatment, your doctor may require you to undergo certain heart tests. If you have any questions or concerns, discuss them with your doctor.
Regularly monitor your blood pressure and heart rate as instructed by your doctor.
If you have diabetes and are taking medications to manage your blood sugar levels, consult your doctor. Weight loss during treatment may increase the risk of hypoglycemia (low blood sugar) in individuals taking diabetes medications. Be aware of the signs of hypoglycemia, which include dizziness, headache, drowsiness, weakness, tremors, rapid heartbeat, confusion, hunger, and sweating. If you experience any of these symptoms, contact your doctor immediately.
Follow the diet and exercise plan recommended by your doctor to ensure safe and effective treatment.
Before consuming alcohol, discuss the potential risks and interactions with your doctor.
This medication is not approved for use in children under 17 years of age.
When starting this medication, be cautious and avoid driving or engaging in activities that require your full attention until you understand how it affects you.
Adhere to your doctor's prescribed treatment duration and dosage instructions. Do not take this medication for longer than recommended, as prolonged use or high doses can lead to tolerance, reducing its effectiveness. If you experience a decrease in the medication's effectiveness, consult your doctor. Avoid taking more than the prescribed dose.
If you have been taking this medication for an extended period, consult your doctor before stopping treatment. Your doctor may recommend a gradual tapering off the medication to minimize potential withdrawal effects. Be aware that this medication can be habit-forming, and long-term use should be avoided. If you have a history of substance or alcohol abuse, inform your doctor.
Before initiating treatment, your doctor may require you to undergo certain heart tests. If you have any questions or concerns, discuss them with your doctor.
Regularly monitor your blood pressure and heart rate as instructed by your doctor.
If you have diabetes and are taking medications to manage your blood sugar levels, consult your doctor. Weight loss during treatment may increase the risk of hypoglycemia (low blood sugar) in individuals taking diabetes medications. Be aware of the signs of hypoglycemia, which include dizziness, headache, drowsiness, weakness, tremors, rapid heartbeat, confusion, hunger, and sweating. If you experience any of these symptoms, contact your doctor immediately.
Follow the diet and exercise plan recommended by your doctor to ensure safe and effective treatment.
Before consuming alcohol, discuss the potential risks and interactions with your doctor.
This medication is not approved for use in children under 17 years of age.
Overdose Information
Overdose Symptoms:
- Restlessness
- Tremor
- Rapid breathing
- Confusion
- Hallucinations
- Panic
- Aggressiveness
- High fever
- Overactive reflexes
- Nausea
- Vomiting
- Diarrhea
- Abdominal cramps
- Irregular heartbeat
- High or low blood pressure
- Circulatory collapse
- Seizures
- Coma
What to Do:
Seek immediate medical attention or call 911. For poison control, call 1-800-222-1222.
Drug Interactions
Contraindicated Interactions
- Monoamine Oxidase Inhibitors (MAOIs) - concurrent use or within 14 days of MAOI discontinuation (risk of hypertensive crisis)
- Other sympathomimetic agents (e.g., other anorexiants, decongestants like pseudoephedrine, ephedrine) - increased risk of cardiovascular effects
- Pulmonary arterial hypertension (PAH) agents (e.g., fenfluramine, dexfenfluramine) - historical association with primary pulmonary hypertension (PPH) when used in combination
Major Interactions
- Alcohol - increased CNS effects and potential for cardiovascular adverse events
- Antidepressants (e.g., SSRIs, SNRIs, TCAs) - potential for serotonin syndrome (especially with SSRIs/SNRIs) or increased pressor effects (with TCAs)
- Insulin and oral hypoglycemic agents - phentermine may alter blood glucose levels, requiring adjustment of antidiabetic medication dosage
- Guanethidine - phentermine may decrease the hypotensive effect of guanethidine
Moderate Interactions
- Antihypertensives (e.g., beta-blockers, ACE inhibitors, diuretics) - phentermine may counteract the hypotensive effects
- Thyroid hormones - may increase CNS stimulation and cardiovascular effects
- Urinary alkalinizers (e.g., sodium bicarbonate, acetazolamide) - may increase phentermine half-life and duration of action
- Urinary acidifiers (e.g., ammonium chloride, ascorbic acid) - may decrease phentermine half-life and duration of action
Minor Interactions
- Caffeine - additive stimulant effects
Monitoring
Baseline Monitoring
Blood Pressure (BP) and Heart Rate (HR)
Rationale: Phentermine is a sympathomimetic and can cause increases in BP and HR. Baseline assessment is crucial to identify pre-existing cardiovascular risk.
Timing: Prior to initiation of therapy
Weight and BMI
Rationale: To establish baseline for efficacy assessment and ensure patient meets criteria for therapy.
Timing: Prior to initiation of therapy
Medical History (especially cardiovascular, psychiatric, substance abuse)
Rationale: To identify contraindications or conditions requiring caution.
Timing: Prior to initiation of therapy
Routine Monitoring
Blood Pressure (BP) and Heart Rate (HR)
Frequency: Regularly (e.g., weekly to monthly) during therapy
Target: Maintain within acceptable limits; individualize based on patient's baseline and comorbidities.
Action Threshold: Significant or sustained increases (e.g., >10-15 mmHg systolic or >10 bpm HR) may warrant dose reduction or discontinuation.
Weight and BMI
Frequency: Monthly
Target: Evidence of weight loss (e.g., 5% weight loss within 12 weeks is a common benchmark for efficacy of obesity medications).
Action Threshold: Lack of adequate weight loss after 12 weeks may indicate non-response and warrant discontinuation.
Mental Status/Behavioral Changes
Frequency: Periodically
Target: Normal mood, absence of agitation, anxiety, or psychotic symptoms.
Action Threshold: New or worsening anxiety, insomnia, agitation, or psychotic symptoms should prompt evaluation and potential discontinuation.
Symptom Monitoring
- Chest pain
- Shortness of breath
- Fainting spells
- Swelling in ankles or feet (signs of pulmonary hypertension)
- Palpitations
- Dizziness
- Severe headache
- Blurred vision
- Agitation
- Anxiety
- Insomnia
- Dry mouth
- Constipation
Special Patient Groups
Pregnancy
Contraindicated in pregnancy (Pregnancy Category X). Weight loss during pregnancy offers no benefit to the pregnant woman and may result in fetal harm. Phentermine has been shown to cause fetal harm in animal studies.
Trimester-Specific Risks:
First Trimester:
Potential for fetal harm; avoid use.
Second Trimester:
Potential for fetal harm; avoid use.
Third Trimester:
Potential for fetal harm; avoid use.
Lactation
Contraindicated during breastfeeding (Lactation Risk L5). Phentermine is excreted into breast milk and could cause serious adverse reactions in a nursing infant (e.g., irritability, sleep disturbances, cardiovascular effects).
Infant Risk:
High risk of adverse effects in the infant; avoid use.
Pediatric Use
Not recommended for children under 16 years of age due to lack of established safety and efficacy. Use in adolescents (16-18 years) should be approached with extreme caution and only if benefits clearly outweigh risks, under strict medical supervision.
Geriatric Use
Use with caution in elderly patients (≥65 years). Elderly patients may be more sensitive to the cardiovascular and CNS stimulant effects of phentermine. Consider starting with lower doses and titrating slowly, and monitor closely for adverse effects, especially cardiovascular and psychiatric.
Clinical Information
Clinical Pearls
- Phentermine is indicated for short-term use (a few weeks) only, typically up to 12 weeks. Its efficacy beyond this period has not been established, and the risk of tolerance and dependence increases with prolonged use.
- It is an adjunct to, not a substitute for, a comprehensive weight management program including diet, exercise, and behavioral modification.
- Patients should be screened for cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, and history of drug abuse prior to initiation.
- Due to its stimulant properties, phentermine should be taken early in the day to avoid insomnia.
- Tolerance to the anorectic effect may develop within a few weeks. If this occurs, the drug should be discontinued rather than increasing the dose.
- Potential for abuse and dependence exists; phentermine is a Schedule IV controlled substance.
Alternative Therapies
- Liraglutide (Saxenda, Victoza)
- Semaglutide (Wegovy, Ozempic)
- Naltrexone/Bupropion (Contrave)
- Orlistat (Xenical, Alli)
- Setmelanotide (Imcivree - for specific genetic conditions)
- Bariatric surgery (for severe obesity)
Cost & Coverage
Average Cost:
$15 - $50 per 30 tablets (37.5mg)
Generic Available:
Yes
Insurance Coverage:
Often covered by most insurance plans, typically as a Tier 1 or Tier 2 generic, but may require prior authorization for obesity management.
General Drug Facts
If your symptoms or health issues persist or worsen, it's essential to contact your doctor for further guidance. To ensure your safety, never share your medication with others or take someone else's medication. Some medications may come with an additional patient information leaflet, so be sure to check with your pharmacist for more information. If you have any questions or concerns about this medication, don't hesitate to reach out to your doctor, nurse, pharmacist, or other healthcare provider. In the event of a suspected overdose, immediately call your local poison control center or seek medical attention. When seeking help, be prepared to provide details about the medication, including the amount taken and the time it happened, to ensure you receive the best possible care.