Keflex (Cephalexin)
Keflex is the original brand name for cephalexin, a first-generation oral cephalosporin antibiotic introduced by Eli Lilly. Used for skin and soft tissue, respiratory tract, bone and urinary tract infections and otitis media. Capsules in 250mg, 500mg and 750mg, plus an oral suspension.
- Active ingredients
- Cephalexin
- Manufacturer
- Eli Lilly
- Dosage forms
- capsule , oral suspension
- Category
- Antibiotics
What is it?
Keflex is the original brand name for cephalexin, an oral first-generation cephalosporin antibiotic approved by the FDA in 1971. It was developed and launched by Eli Lilly, and although the brand has since changed hands in the United States, the Keflex name remains familiar to prescribers and patients. Cephalexin is one of the most widely prescribed oral antibiotics in the United States and United Kingdom, and generic cephalexin has been available for decades at low cost. The molecule is on the WHO List of Essential Medicines.
Active ingredients
Each Keflex capsule contains cephalexin monohydrate, expressed as cephalexin, in 250mg, 500mg or 750mg strengths; the oral suspension provides 125mg or 250mg per 5mL once reconstituted. Cephalexin is a beta-lactam antibiotic that binds penicillin-binding proteins in the bacterial cell wall and halts peptidoglycan synthesis. It is active against most streptococci, methicillin-susceptible Staphylococcus aureus and some Gram-negative organisms such as E. coli, Proteus mirabilis and Klebsiella, but not against MRSA, enterococci or Pseudomonas.
Forms and dosages
Keflex is supplied as hard capsules (250mg, 500mg and 750mg) and as a powder for oral suspension for children or adults who cannot swallow capsules. The usual adult dose is 250mg every 6 hours or 500mg every 12 hours, increased to 500mg every 6 hours for more severe infections, with a maximum of 4g per day. According to the prescribing information, courses typically run 7 to 14 days, and the dose must be reduced in patients with a creatinine clearance below 60mL/min. Capsules may be taken with or without food.
Indications
Keflex is approved for respiratory tract infections caused by Streptococcus pneumoniae and Streptococcus pyogenes, otitis media, skin and skin-structure infections caused by staphylococci and streptococci, bone infections, and genitourinary infections including acute prostatitis. In practice it is a first-line option for uncomplicated cellulitis, impetigo and streptococcal pharyngitis in penicillin-tolerant patients, and a common choice for uncomplicated cystitis. According to clinical guidelines, a culture should be considered before starting therapy when the pathogen is uncertain, and a doctor should reassess if there is no improvement within 48 to 72 hours.
How it works
Cephalexin is bactericidal: it acylates penicillin-binding proteins in the bacterial cell wall, blocking the final cross-linking step of peptidoglycan synthesis. The weakened wall cannot withstand osmotic pressure and the bacterium lyses while dividing. Cephalexin is rapidly absorbed from the gut with almost complete bioavailability, reaches peak plasma concentration in about one hour, and has a half-life of roughly one hour in adults with normal kidney function. It is excreted unchanged in the urine, which gives high urinary concentrations and explains its usefulness in urinary tract infections.
Frequently asked questions
Can I take Keflex if I am allergic to penicillin? ▾
It depends on the type of reaction. Cephalexin shares a beta-lactam ring with penicillins, and historic estimates of cross-allergy were around 10%, but modern studies put the true figure at roughly 1 to 3%. People who had a mild, non-immediate rash to penicillin can often take cephalexin safely. Anyone with a history of anaphylaxis, angioedema or severe skin reactions to a penicillin should not take Keflex without an allergist's assessment. Always tell the prescribing doctor about any previous antibiotic reaction.
How quickly does Keflex start working? ▾
Cephalexin reaches peak blood levels within about an hour of a dose, and most patients notice improvement in symptoms such as fever, pain or redness within 48 to 72 hours. The full course should still be completed even when symptoms resolve early, because stopping too soon allows surviving bacteria to regrow and encourages resistance. According to the prescribing information, if there is no improvement after three days the doctor should reassess the diagnosis and consider a different antibiotic.
What are the most common side effects of Keflex? ▾
The most frequent side effects are gastrointestinal: diarrhoea, nausea, abdominal discomfort and occasionally vomiting. Vaginal yeast infections and mild skin rash are also reported. Rarely, cephalexin can cause Clostridioides difficile-associated diarrhoea, which may appear during or up to two months after treatment; severe or watery diarrhoea with abdominal cramps should be reported to a doctor immediately and not self-treated with anti-diarrhoeal drugs.
Can I drink alcohol while taking Keflex? ▾
Cephalexin does not cause the disulfiram-like reaction seen with some other cephalosporins such as cefotetan, so moderate alcohol is not strictly contraindicated. Alcohol can, however, worsen nausea and stomach upset, slow recovery and cause dehydration, which matters in urinary tract infections. Most doctors recommend avoiding or limiting alcohol until the course is finished and you feel well.
Does Keflex interact with other medicines? ▾
Clinically significant interactions are few. Probenecid reduces renal excretion and raises cephalexin levels. Metformin concentrations may increase slightly when the two are taken together. Like other antibiotics, cephalexin can enhance the effect of warfarin, so INR monitoring is sensible on longer courses. There is no meaningful interaction with hormonal contraceptives, although diarrhoea or vomiting may reduce pill absorption.
Is Keflex safe during pregnancy and breastfeeding? ▾
Cephalexin has a long record of use in pregnancy and is one of the antibiotics commonly chosen for urinary tract infections in pregnant women; it is considered compatible with breastfeeding, as only small amounts pass into milk. As with any medicine in pregnancy, it should be taken only when prescribed by a doctor who knows the full clinical picture.
Related medications
Sources
This page is compiled from the official prescribing information for Cephalexin. The links below open a search for that active ingredient in each register, so you always reach the current version of the label rather than a cached copy.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.