Cephalexin

Cephalexin is a first-generation cephalosporin antibiotic, part of the broader beta-lactam family that also includes penicillins and carbapenems ^[1]. Like all beta-lactams, cephalexin kills bacteria by interfering with their ability to build and maintain cell walls.

Specifically, cephalexin binds to penicillin-binding proteins (PBPs) — enzymes located on the inner surface of the bacterial cell membrane that are essential for the final cross-linking step in peptidoglycan synthesis ^[2]. Peptidoglycan is the mesh-like polymer that gives the bacterial cell wall its structural integrity. When cephalexin inhibits the transpeptidase activity of PBPs, the cross-links between peptidoglycan strands cannot form, resulting in a weakened cell wall ^[1].

As the bacterium continues to grow and divide with a defective cell wall, internal osmotic pressure causes the cell to swell and eventually lyse (burst) ^[2]. This makes cephalexin bactericidal — it actively kills bacteria rather than merely inhibiting their growth.

As a first-generation cephalosporin, cephalexin has its strongest activity against gram-positive organisms including *Staphylococcus aureus* (methicillin-sensitive), *Streptococcus pyogenes*, and *Streptococcus pneumoniae* ^[1][3]. It also covers some common gram-negative organisms such as *E. coli*, *Klebsiella pneumoniae*, and *Proteus mirabilis*, though second- and third-generation cephalosporins provide broader gram-negative coverage.

Cephalexin is generally well tolerated and starts working quickly. Most patients with skin infections notice redness and swelling beginning to improve within 48-72 hours ^[1].

The most common side effects are mild GI symptoms — diarrhea, nausea, and stomach upset occur in about 5-10% of patients. These are usually manageable by taking the medication with food ^[1].

If you have a penicillin allergy, inform your doctor. Cross-reactivity between penicillins and first-generation cephalosporins is estimated at 1-2% (much lower than previously thought), and cephalexin can often be safely used in patients with non-severe penicillin allergies after careful evaluation ^[3].

Women may develop a vaginal yeast infection during or after antibiotic treatment due to disruption of normal flora. Over-the-counter antifungal treatments are effective if this occurs.

Complete the full prescribed course even if you feel better. Stopping early contributes to antibiotic resistance.

Food: Cephalexin can be taken with or without food ^[1]. Food may slightly delay but does not reduce the total absorption of the drug. Taking cephalexin with food may help reduce mild GI side effects like nausea and stomach upset.

Dairy Products: Unlike fluoroquinolones and tetracyclines, cephalexin is not affected by dairy products or calcium-containing foods ^[1]. You can freely consume milk, yogurt, and cheese while taking cephalexin.

Alcohol: There is no specific interaction between cephalexin and alcohol, and cephalexin does not cause a disulfiram-like reaction ^[2]. However, alcohol may worsen GI side effects and impair immune function during active infection. Moderate alcohol use during treatment is generally considered acceptable, though avoiding alcohol during any active infection is prudent.

Probiotics: Some clinicians recommend taking probiotics (or yogurt with live cultures) during antibiotic treatment to reduce the risk of antibiotic-associated diarrhea. If using probiotics, take them at least 2 hours apart from the antibiotic dose ^[3].

Skin and Soft Tissue Infections: 250-500 mg orally every 6 hours for 7-14 days ^[1]. For mild cellulitis in adults, 500 mg four times daily is typical.

Streptococcal Pharyngitis: 250 mg orally every 6 hours or 500 mg every 12 hours for 10 days ^[1].

Urinary Tract Infections (uncomplicated): 250-500 mg every 6 hours for 7-14 days ^[1]. Note: cephalexin is not considered first-line for uncomplicated UTI by IDSA guidelines.

Bone Infections: 500 mg to 1 g every 6 hours for 4-6 weeks, often as oral step-down therapy after initial IV antibiotics ^[1].

Otitis Media: 75-100 mg/kg/day in children divided into 4 doses for 10 days ^[1].

Pediatric Dosing: 25-50 mg/kg/day in divided doses every 6-12 hours; up to 100 mg/kg/day for severe infections ^[1].

Renal Adjustment: CrCl 10-50 mL/min: no adjustment or extend interval to every 8-12 hours. CrCl <10 mL/min: 250 mg every 12-24 hours ^[1].

Maximum dose: 4 g/day in adults.

Generic cephalexin is one of the most affordable antibiotics available. A typical 10-day course of 500 mg capsules (QID) costs approximately $8-15 at most pharmacies ^[4].

Cost-saving strategies: - Generic is universal: Brand-name Keflex has been effectively discontinued; all prescriptions are filled with generic cephalexin, which has been available since the 1980s - $4 generic programs: Cephalexin is included on Walmart, Kroger, and most other pharmacy $4/$10 generic drug lists - Capsules vs. tablets: Capsules and tablets are therapeutically equivalent; choose whichever is cheapest at your pharmacy - Oral suspension for children: Generic cephalexin suspension (125 mg/5 mL or 250 mg/5 mL) is similarly affordable at $8-15 - GoodRx coupons: Can reduce out-of-pocket costs to $4-8 for a typical course - Twice-daily dosing: For some indications, 500 mg BID may be as effective as 250 mg QID, improving compliance without additional cost

Pregnancy (Category B): Cephalexin crosses the placenta but has a strong safety record in pregnancy ^[1]. Animal reproduction studies have not shown fetal harm, and extensive human experience over decades supports its safety. Cephalexin is one of the preferred antibiotics during pregnancy for conditions such as UTIs, skin infections, and Group B Streptococcus prophylaxis when penicillin cannot be used ^[5].

Breastfeeding: Cephalexin is excreted in breast milk in small amounts. The AAP considers it compatible with breastfeeding ^[6]. Infant exposure through breast milk is estimated at less than 1% of the maternal dose. The main concern is potential disruption of infant gut flora, which may manifest as diarrhea or oral thrush in the nursing infant. These effects are generally mild and self-limiting.

Pediatric Use: Cephalexin is FDA-approved for children of all ages and is one of the most commonly prescribed antibiotics in pediatrics ^[1]. Oral suspension formulations are available for young children.

Brand-name Keflex has been effectively discontinued by its original manufacturer, and virtually all cephalexin prescriptions in the United States are filled with generic equivalents ^[1]. Generic cephalexin has been available since the 1980s and is manufactured by dozens of companies.

All generic versions are FDA "AB" rated as therapeutically equivalent. Cephalexin is available as capsules (250 mg, 500 mg), tablets (250 mg, 500 mg), and oral suspension (125 mg/5 mL, 250 mg/5 mL).

There is absolutely no clinical advantage to seeking brand-name Keflex over generic cephalexin. The generic is a commodity antibiotic with extremely stable pricing.