Keflex 500mg Capsules
It is used to treat bacterial infections.
Drug Class
Anti-infective
Pharmacologic Class
First-generation cephalosporin antibiotic
Pregnancy Category
Category B
FDA Approved
Jan 1971
DEA Schedule
Not Controlled
Overview
What is this medicine?
Cephalexin is an antibiotic medication used to treat various bacterial infections, such as those affecting the skin, ears, throat, and urinary tract. It works by stopping the growth of bacteria.
How to Use This Medicine
Taking Your Medication
To use this medication correctly, follow your doctor's instructions and read all the information provided. Take your medication as directed, with or without food. If you experience stomach upset, taking it with food may help. Continue using the medication even if your symptoms improve.
Storing and Disposing of Your Medication
Store your medication at room temperature in a dry place, avoiding the bathroom. Keep all medications in a safe location, out of the reach of children and pets. Dispose of unused or expired medications properly. Do not flush them down the toilet or pour them down the drain unless instructed to do so. If you have questions about disposing of your medication, consult your pharmacist. You may also want to check if there are drug take-back programs in your area.
Missing 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 dose, skip the missed dose and resume your regular schedule. Do not take two doses at the same time or take extra doses to make up for the missed one.
To use this medication correctly, follow your doctor's instructions and read all the information provided. Take your medication as directed, with or without food. If you experience stomach upset, taking it with food may help. Continue using the medication even if your symptoms improve.
Storing and Disposing of Your Medication
Store your medication at room temperature in a dry place, avoiding the bathroom. Keep all medications in a safe location, out of the reach of children and pets. Dispose of unused or expired medications properly. Do not flush them down the toilet or pour them down the drain unless instructed to do so. If you have questions about disposing of your medication, consult your pharmacist. You may also want to check if there are drug take-back programs in your area.
Missing 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 dose, skip the missed dose and resume your regular schedule. Do not take two doses at the same time or take extra doses to make up for the missed one.
Lifestyle & Tips
- Take the medication exactly as prescribed by your doctor, even if you start to feel better. Do not skip doses or stop taking it early, as this can lead to antibiotic resistance.
- Cephalexin can be taken with or without food. If it causes stomach upset, try taking it with food or milk.
- Drink plenty of fluids while taking this medication.
- Avoid alcohol, as it may worsen stomach upset.
- Do not share this medication with others, even if they have similar symptoms.
Available Forms & Alternatives
Available Strengths:
Generic Alternatives:
Dosing & Administration
Adult Dosing
Standard Dose:
500 mg every 12 hours for mild to moderate infections (e.g., skin and soft tissue, uncomplicated UTI)
Dose Range:
250 - 1000 mg
Condition-Specific Dosing:
mildToModerateInfections:
250 mg to 500 mg every 6 hours or 500 mg every 12 hours
severeInfections:
500 mg to 1 g every 6 hours
streptococcalPharyngitis:
250 mg every 6 hours or 500 mg every 12 hours for 10 days
skinAndSoftTissueInfections:
500 mg every 12 hours
Pediatric Dosing
Neonatal:
Not established (use caution, consult specific guidelines)
Infant:
25-50 mg/kg/day in divided doses every 6-12 hours (max 100 mg/kg/day)
Child:
25-50 mg/kg/day in divided doses every 6-12 hours (max 100 mg/kg/day, up to 4 g/day)
Adolescent:
Same as adult dosing, typically 25-50 mg/kg/day in divided doses, or adult dose if weight allows
Dose Adjustments
Renal Impairment:
Mild:
No adjustment typically needed (CrCl > 50 mL/min)
Moderate:
500 mg every 8-12 hours (CrCl 30-49 mL/min)
Severe:
500 mg every 12-24 hours (CrCl 10-29 mL/min)
Dialysis:
500 mg every 24-48 hours, with a supplemental dose of 500 mg after each dialysis session
Hepatic Impairment:
Mild:
No adjustment
Moderate:
No adjustment
Severe:
No adjustment
Pharmacology
Mechanism of Action
Cephalexin is a first-generation cephalosporin antibiotic that inhibits bacterial cell wall synthesis by binding to one or more of the penicillin-binding proteins (PBPs). This inhibits the final transpeptidation step of peptidoglycan synthesis in the bacterial cell wall, leading to bacterial cell lysis and death.
Pharmacokinetics
Absorption:
Bioavailability:
Approximately 90%
Tmax:
1 hour (fasted)
FoodEffect:
Food may delay the rate of absorption (Tmax) but does not significantly affect the extent of absorption (AUC).
Distribution:
Vd:
Approximately 0.26 L/kg
ProteinBinding:
Approximately 10-15%
CnssPenetration:
Limited (does not achieve therapeutic concentrations in CSF unless meninges are inflamed)
Elimination:
HalfLife:
0.5 to 1.2 hours (normal renal function)
Clearance:
Approximately 210 mL/min (renal clearance)
ExcretionRoute:
Renal (primarily via glomerular filtration and tubular secretion)
Unchanged:
>90%
Pharmacodynamics
OnsetOfAction:
Rapid (within hours of first dose)
PeakEffect:
Within 1-2 hours of administration
DurationOfAction:
Dependent on dosing interval (typically 6-12 hours due to short half-life)
Safety & Warnings
Side Effects
Urgent Side Effects: Seek Medical Help Right Away
While rare, some people may experience severe and potentially life-threatening side effects when taking this medication. If you notice any of the following symptoms, contact your doctor or seek medical attention immediately:
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 a severe skin reaction (Stevens-Johnson syndrome/toxic epidermal necrolysis), such as:
+ Red, swollen, blistered, or peeling skin (with or without fever)
+ Red or irritated eyes
+ Sores in the mouth, throat, nose, or eyes
Signs of liver problems, including:
+ Dark urine
+ Tiredness
+ Decreased appetite
+ Upset stomach or stomach pain
+ Light-colored stools
+ Vomiting
+ Yellow skin or eyes
Fever, chills, or sore throat
Unexplained bruising or bleeding
Feeling extremely tired or weak
Confusion
Hallucinations (seeing or hearing things that are not there)
Difficulty urinating or changes in urine output
Seizures
Severe dizziness
Severe headache
Severe joint pain
Vaginal itching or discharge
Antibiotic-Associated Diarrhea
Diarrhea is a common side effect of antibiotics. In rare cases, a severe form of diarrhea called C. diff-associated diarrhea (CDAD) may occur. This can lead to a life-threatening bowel problem. If you experience any of the following symptoms, contact your doctor immediately:
Stomach pain
Cramps
Very loose, watery, or bloody stools
Do not treat diarrhea without consulting your doctor first.
Other Possible Side Effects
Like all medications, this drug can cause side effects. Many people experience no side effects or only mild ones. If you notice any of the following side effects or any other symptoms that bother you or do not go away, contact your doctor:
Stomach pain or diarrhea
* Upset stomach or vomiting
This is not a comprehensive list of all possible side effects. If you have questions or concerns about side effects, contact your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
While rare, some people may experience severe and potentially life-threatening side effects when taking this medication. If you notice any of the following symptoms, contact your doctor or seek medical attention immediately:
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 a severe skin reaction (Stevens-Johnson syndrome/toxic epidermal necrolysis), such as:
+ Red, swollen, blistered, or peeling skin (with or without fever)
+ Red or irritated eyes
+ Sores in the mouth, throat, nose, or eyes
Signs of liver problems, including:
+ Dark urine
+ Tiredness
+ Decreased appetite
+ Upset stomach or stomach pain
+ Light-colored stools
+ Vomiting
+ Yellow skin or eyes
Fever, chills, or sore throat
Unexplained bruising or bleeding
Feeling extremely tired or weak
Confusion
Hallucinations (seeing or hearing things that are not there)
Difficulty urinating or changes in urine output
Seizures
Severe dizziness
Severe headache
Severe joint pain
Vaginal itching or discharge
Antibiotic-Associated Diarrhea
Diarrhea is a common side effect of antibiotics. In rare cases, a severe form of diarrhea called C. diff-associated diarrhea (CDAD) may occur. This can lead to a life-threatening bowel problem. If you experience any of the following symptoms, contact your doctor immediately:
Stomach pain
Cramps
Very loose, watery, or bloody stools
Do not treat diarrhea without consulting your doctor first.
Other Possible Side Effects
Like all medications, this drug can cause side effects. Many people experience no side effects or only mild ones. If you notice any of the following side effects or any other symptoms that bother you or do not go away, contact your doctor:
Stomach pain or diarrhea
* Upset stomach or vomiting
This is not a comprehensive list of all possible side effects. If you have questions or concerns about side effects, contact 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:
- Signs of a severe allergic reaction: hives, difficulty breathing, swelling of your face, lips, tongue, or throat.
- Severe stomach pain, watery or bloody diarrhea (even if it occurs months after your last dose).
- Easy bruising or bleeding, unusual weakness.
- Fever, chills, body aches, flu symptoms.
- Pale skin, light-headedness, shortness of breath, rapid heart rate.
- Dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).
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:
Any allergies you have, including allergies to this medication, its components, or other substances, such as foods or drugs. Be sure to describe the allergic reaction you experienced, including any symptoms that occurred.
If you are currently taking probenecid, as this may interact with the medication.
Additionally, to ensure safe treatment, it is crucial to provide your doctor and pharmacist with a comprehensive list of:
All prescription and over-the-counter (OTC) medications you are taking
Any natural products or vitamins you are using
* Your existing health problems
This information will help your doctor determine whether it is safe to take this medication in conjunction with your other medications and health conditions. Never start, stop, or adjust the dosage of any medication without first consulting your doctor to confirm that it is safe to do so.
It is essential to inform your doctor about the following:
Any allergies you have, including allergies to this medication, its components, or other substances, such as foods or drugs. Be sure to describe the allergic reaction you experienced, including any symptoms that occurred.
If you are currently taking probenecid, as this may interact with the medication.
Additionally, to ensure safe treatment, it is crucial to provide your doctor and pharmacist with a comprehensive list of:
All prescription and over-the-counter (OTC) medications you are taking
Any natural products or vitamins you are using
* Your existing health problems
This information will help your doctor determine whether it is safe to take this medication in conjunction with your other medications and health conditions. Never start, stop, or adjust the dosage of any medication without first consulting your doctor to confirm that it is safe to do so.
Precautions & Cautions
It is essential to inform all your healthcare providers, including doctors, nurses, pharmacists, and dentists, that you are taking this medication. If you are taking this drug for an extended period, your doctor may recommend regular blood tests to monitor your condition.
If you have diabetes and regularly test your urine for glucose, consult with your doctor to determine the most suitable testing method for you, as this medication may interfere with certain test results. Additionally, be sure to notify all your healthcare providers and laboratory personnel that you are taking this medication, as it may affect the outcomes of certain lab tests.
Do not take this medication for longer than prescribed, as this can increase the risk of developing a secondary infection.
If you are pregnant, planning to become pregnant, or are breastfeeding, it is crucial to discuss the potential benefits and risks of this medication with your doctor to ensure the best possible outcome for you and your baby.
If you have diabetes and regularly test your urine for glucose, consult with your doctor to determine the most suitable testing method for you, as this medication may interfere with certain test results. Additionally, be sure to notify all your healthcare providers and laboratory personnel that you are taking this medication, as it may affect the outcomes of certain lab tests.
Do not take this medication for longer than prescribed, as this can increase the risk of developing a secondary infection.
If you are pregnant, planning to become pregnant, or are breastfeeding, it is crucial to discuss the potential benefits and risks of this medication with your doctor to ensure the best possible outcome for you and your baby.
Overdose Information
Overdose Symptoms:
- Nausea
- Vomiting
- Epigastric distress
- Diarrhea
- Hematuria (rare)
- In rare cases, seizures (especially in patients with renal impairment)
What to Do:
In case of suspected overdose, seek immediate medical attention or call Poison Control at 1-800-222-1222. Management is primarily supportive, including gastric lavage or activated charcoal if ingestion is recent. Hemodialysis may be useful in severe cases, especially with renal impairment.
Drug Interactions
Major Interactions
- Live bacterial vaccines (e.g., typhoid vaccine, oral): May reduce therapeutic effect of the vaccine.
Moderate Interactions
- Metformin: Cephalexin may increase metformin concentrations by inhibiting organic cation transporter 2 (OCT2) in the kidney, potentially increasing risk of lactic acidosis.
- Probenecid: Decreases renal tubular secretion of cephalexin, resulting in increased and prolonged serum concentrations of cephalexin.
- Oral anticoagulants (e.g., Warfarin): May enhance the anticoagulant effect by altering gut flora, though this is rare and generally not clinically significant for short courses.
- Cholera vaccine: May reduce the therapeutic effect of the vaccine.
Monitoring
Baseline Monitoring
Culture and Sensitivity
Rationale: To confirm susceptibility of the causative organism to cephalexin, especially for serious infections.
Timing: Prior to initiation of therapy, if clinically indicated.
Renal Function (BUN, Creatinine)
Rationale: To assess baseline renal function, as cephalexin is primarily renally eliminated and dose adjustments are required in renal impairment.
Timing: Prior to initiation, especially in elderly patients or those with known or suspected renal impairment.
Routine Monitoring
Clinical Response to Therapy
Frequency: Daily
Target: Resolution of signs and symptoms of infection
Action Threshold: Lack of improvement or worsening symptoms may indicate treatment failure, resistance, or alternative diagnosis.
Signs of Superinfection (e.g., oral thrush, vaginal candidiasis, C. difficile-associated diarrhea)
Frequency: Daily throughout therapy and for several weeks post-therapy
Target: Absence of new infections
Action Threshold: Appearance of new symptoms requires evaluation and appropriate management.
Renal Function (BUN, Creatinine)
Frequency: Periodically, especially in prolonged therapy or in patients with impaired renal function
Target: Stable or improving renal function
Action Threshold: Significant decline in renal function may necessitate dose adjustment or discontinuation.
Symptom Monitoring
- Allergic reactions (rash, itching, hives, swelling of face/lips/tongue, difficulty breathing)
- Severe or watery diarrhea (may indicate Clostridioides difficile-associated diarrhea)
- Nausea, vomiting, abdominal pain
- Unusual tiredness or weakness
- Unusual bleeding or bruising
- Signs of new infection (e.g., fever, sore throat, chills)
Special Patient Groups
Pregnancy
Cephalexin is generally considered safe for use during pregnancy (Pregnancy Category B). Animal reproduction studies have not demonstrated a risk to the fetus, but there are no adequate and well-controlled studies in pregnant women.
Trimester-Specific Risks:
First Trimester:
No increased risk of major birth defects observed in human studies.
Second Trimester:
Generally considered safe.
Third Trimester:
Generally considered safe.
Lactation
Cephalexin is excreted in breast milk in low concentrations. It is generally considered compatible with breastfeeding (Lactation Risk L2 - Safer).
Infant Risk:
Low risk of adverse effects in breastfed infants. Monitor for potential changes in bowel flora (e.g., diarrhea, candidiasis) or allergic reactions (e.g., rash).
Pediatric Use
Dosing is weight-based and should be carefully calculated. Safety and efficacy have been established for pediatric patients. Neonatal use requires caution and specific guidelines due to immature renal function.
Geriatric Use
No overall differences in safety or effectiveness have been observed between elderly and younger patients, but greater sensitivity of some older individuals cannot be ruled out. Dose adjustment is necessary in elderly patients with impaired renal function.
Clinical Information
Clinical Pearls
- Cephalexin is a first-generation cephalosporin, effective against many Gram-positive bacteria (e.g., Staphylococcus aureus, Streptococcus pyogenes) and some Gram-negative bacteria (e.g., E. coli, Klebsiella pneumoniae, Proteus mirabilis).
- It is a common choice for skin and soft tissue infections (e.g., cellulitis, impetigo), uncomplicated urinary tract infections, and streptococcal pharyngitis.
- Not effective against MRSA (Methicillin-resistant Staphylococcus aureus) or most enterococci.
- Good oral bioavailability means it's well-absorbed and can often be used for outpatient treatment or as step-down therapy from IV antibiotics.
- Taking with food can help minimize gastrointestinal upset (nausea, vomiting, diarrhea).
Alternative Therapies
- Other first-generation cephalosporins (e.g., Cefazolin IV)
- Penicillins (e.g., Amoxicillin, Dicloxacillin, Penicillin V)
- Macrolides (e.g., Azithromycin, Clarithromycin, Erythromycin) for penicillin-allergic patients
- Clindamycin (for skin/soft tissue infections, including some MRSA strains)
- Trimethoprim/Sulfamethoxazole (for UTIs, some skin infections)
- Fluoroquinolones (e.g., Ciprofloxacin, Levofloxacin) for specific indications
Cost & Coverage
Average Cost:
Highly variable, typically low for generic per 30 capsules
Generic Available:
Yes
Insurance Coverage:
Tier 1 (Preferred Generic)
General Drug Facts
If your symptoms or health issues persist or worsen, it is 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 it's a good idea 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 emergency medical attention. When seeking help, be prepared to provide details about the overdose, including the medication taken, the amount, and the time it occurred.