Keflex: Uses, dosage, side effects, warnings & patient reviews

What is Keflex (Cephalexin)?

Keflex is a prescription antibiotic containing the active ingredient cephalexin. It belongs to the cephalosporin class of antibiotics, which are structurally related to penicillins; however, most people with penicillin allergy can take cephalexin safely under medical guidance. Keflex works by interfering with the formation of the bacterial cell wall, leading to the death of susceptible bacteria.

It is considered a first-generation cephalosporin, effective against a range of common Gram-positive bacteria and some Gram-negative bacteria. Keflex is available in several forms, including capsules (commonly 250 mg and 500 mg), tablets, and an oral suspension for children.

A common and notable characteristic of cephalexin is its distinctive sulfur-like odor, which is natural and does not indicate that the medication is spoiled or ineffective.

How to Take Keflex

Following the prescribed dosing schedule is crucial for the effectiveness of Keflex and to prevent antibiotic resistance.

Administration and Timing

Keflex capsules, tablets, or liquid suspension are taken by mouth. The medication can be taken with or without food. Taking it with food may help reduce stomach upset.

The typical dosing frequency is two to four times daily, depending on the infection’s severity. It is important to space doses evenly throughout the day (e.g., every 6 or 12 hours) to maintain a constant level of the drug in your body. Setting reminders or alarms can help ensure you do not miss a dose.

Course Completion and Missed Dose

Continue taking Keflex for the full prescribed length of time, even if symptoms disappear after a few days. Stopping the medication early may allow the infection to return and can contribute to antibiotic resistance.

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and resume your regular schedule. Do not take a double dose to make up for a missed one.

Special Note for Liquid Suspension

If using the oral suspension, shake the bottle well before measuring each dose. Use the provided measuring spoon or syringe for accuracy. Store the reconstituted suspension as directed by your pharmacist, usually in the refrigerator, and discard any unused portion after the specified number of days.

Side Effects of Keflex

Most people tolerate Keflex well, but like all medications, it can cause side effects. For a general understanding of medication side effects, see our guide to antibiotics.

What is Keflex Used For?

Keflex is prescribed to treat bacterial infections caused by susceptible organisms. It is not effective against viral infections like the common cold or flu.

Primary Medical Uses

• Skin and Soft Tissue Infections: Such as cellulitis, impetigo, and furunculosis (boils). It is a common first-line treatment for uncomplicated skin infections. Learn more about minor skin infections.
• Respiratory Tract Infections: Including pharyngitis (strep throat), tonsillitis, and mild cases of bronchitis caused by susceptible bacteria.
• Bone Infections (Osteomyelitis): Caused by susceptible staphylococci.
• Urinary Tract Infections (UTIs): Such as cystitis (bladder infection). For more on this condition, see information on cystitis.
• Otitis Media (Middle Ear Infection): In children, when caused by susceptible bacteria.

Limitations and Important Notes

Keflex is a first-generation cephalosporin. Its spectrum of activity makes it suitable for many common community-acquired infections, but it may not be appropriate for more serious or hospital-acquired infections resistant to other antibiotics. A healthcare provider will determine if Keflex is the right choice based on the suspected or confirmed type of bacteria.

Important Safety Concerns and Considerations

Allergic Reactions and Cross-Reactivity

There is a risk of allergic reaction to Keflex, ranging from mild rash to severe anaphylaxis. There is an approximately 5-10% cross-reactivity rate in patients with a penicillin allergy. If you have a history of a severe or immediate-type allergy to penicillins (like amoxicillin), use Keflex with caution and only under direct medical supervision. Inform any healthcare provider of all drug allergies before receiving treatment.

Clostridioides difficile-Associated Diarrhea (CDAD)

All antibiotics, including Keflex, can disrupt the normal bacteria in the colon, potentially allowing overgrowth of C. difficile. This can cause mild diarrhea to life-threatening colitis. CDAD can occur during treatment or weeks after the last dose. Report severe, persistent, or bloody diarrhea to your doctor immediately. Do not treat it with anti-diarrheal medications without medical advice.

Kidney Function and Dosing

Cephalexin is primarily eliminated from the body by the kidneys. In patients with significant renal impairment, the drug can accumulate, increasing the risk of side effects, particularly seizures. Dosage adjustment is necessary based on creatinine clearance. Your doctor will determine the appropriate dose if you have kidney issues.

Use in Patients with Gastrointestinal Disease

Cephalexin should be used with caution in patients with a history of colitis, particularly antibiotic-associated colitis, as it may increase the risk of recurrence.

False Positive Urine Glucose Tests

Cephalexin may cause a false-positive reaction for glucose in the urine with certain non-enzymatic tests (e.g., Clinitest). It does not affect glucose oxidase tests (e.g., Clinistix, Tes-Tape). Inform laboratory personnel you are taking cephalexin if providing a urine sample.

Critical Warnings for Keflex Use

Anaphylaxis and Hypersensitivity

Serious and occasionally fatal hypersensitivity (anaphylactic) reactions have been reported with cephalosporins, including Keflex. These reactions are more likely in individuals with a history of allergy, particularly to penicillin. Discontinue Keflex immediately if an allergic reaction occurs and institute emergency treatment (e.g., epinephrine, corticosteroids).

CDAD Diagnosis and Management

If CDAD is suspected or confirmed, Keflex use may need to be discontinued. Appropriate fluid and electrolyte management, protein supplementation, and antibiotic treatment for C. difficile should be initiated. Anti-peristaltic agents (e.g., opioids) may worsen the condition and should be avoided.

Direct Coombs’ Test Seroconversion

Positive direct Coombs’ tests have been reported during treatment with cephalosporins, which can interfere with cross-matching of blood. Inform your hematologist or the blood bank if you are taking Keflex.

Risk of Seizure

Seizures have been reported with cephalosporins, particularly in patients with renal impairment when the dosage was not reduced. Administer Keflex with caution in patients with a history of seizure disorder.

Development of Drug-Resistant Bacteria

Prescribing antibiotics like Keflex in the absence of a proven or strongly suspected bacterial infection is unlikely to provide benefit and increases the risk of developing drug-resistant bacteria. Always use this drug only for the infection for which it was prescribed.

Keflex Dosage Information

Dosage is determined by the treating physician based on the type and severity of infection, the causative organism, and the patient’s renal function. The following are general guidelines for adults with normal renal function.

Important Administration Notes

• Renal Impairment: Dosage adjustment is required. The following is a general guideline, but the physician will determine the exact regimen:
  • Creatinine clearance 10-50 mL/min: 500 mg every 8-12 hours.
  • Creatinine clearance <10 mL/min: 250 or 500 mg every 12-24 hours.
• Dosing Frequency: The total daily dose is usually divided into 2 to 4 equal doses to maintain effective antibiotic levels.

Drug Interactions with Keflex

Keflex has a relatively low potential for significant drug interactions, but a few important ones exist.

Additional Information

Storage and Handling

Store Keflex capsules and tablets at room temperature, away from light, excess heat, and moisture. Keep the oral suspension in the refrigerator after mixing and discard any unused portion after the specified number of days (usually 10-14 days). Keep all medications out of the reach of children and pets.

Mechanism of Action

Cephalexin is a bactericidal antibiotic. It inhibits the synthesis of the bacterial cell wall by binding to specific penicillin-binding proteins (PBPs). This action leads to the formation of a defective cell wall that is unstable, ultimately causing the bacterial cell to lyse (break open) and die.

Reporting Side Effects

You are encouraged to report negative side effects of prescription drugs to the relevant national health authority (e.g., the FDA in the United States). Reporting helps health authorities monitor drug safety and identify new risks.

References and Medical Sources

1. U.S. Food and Drug Administration (FDA). Keflex (cephalexin) Prescribing Information. [Updated 2016]. https://www.accessdata.fda.gov/drugsatfda_docs/label/2018/050405s107lbl.pdf
2. MedlinePlus. Cephalexin. U.S. National Library of Medicine; [Updated 2022]. https://medlineplus.gov/druginfo/meds/a682733.html
3. Drugs.com. Cephalexin Professional Monograph. Drugs.com; [Updated 2024]. https://www.drugs.com/monograph/cephalexin.html
4. Johns Hopkins ABX Guide. Cephalexin. [Accessed 2024].

Disclaimer: The information on this site is provided for informational purposes only and is not medical advice. It does not replace professional medical consultation, diagnosis, or treatment. Do not self-medicate based on the information presented on this site. Always consult with a doctor or other qualified healthcare professional before making any decisions about your health.