How the dose is calculated
Cephalexin, a first-generation cephalosporin, is dosed in mg/kg/day divided across the selected frequency. Mild-to-moderate infections use a lower total daily dose split three times a day; more severe infections use a higher total daily dose split four times a day to maintain adequate trough levels.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Mild-moderate | 25 mg/kg/day | every 8h (3×/day) | 1500 mg/day |
| Severe infection | 50 mg/kg/day | every 6h (4×/day) | 4000 mg/day |
Clinical use
- Cephalexin is a first-generation cephalosporin, active against most streptococci and methicillin-susceptible Staphylococcus aureus — a common first-line choice for uncomplicated skin and soft-tissue infections.
- Around 1–3% of penicillin-allergic patients may cross-react with cephalosporins; use caution (and consider alternatives) in patients with a history of severe/anaphylactic penicillin reaction.
- Not active against MRSA, Pseudomonas, or most Gram-negative organisms beyond E. coli/Proteus/Klebsiella — narrow accordingly for suspected resistant organisms.
- Requires dose-interval adjustment in significant renal impairment.
Frequently asked questions
What is cephalexin used for?
Cephalexin is a first-generation cephalosporin antibiotic commonly used for skin and soft-tissue infections, streptococcal throat infections, and uncomplicated urinary tract infections. A common adult dose is cephalexin 500 mg, typically given every 6–8 hours depending on the infection.
Cephalexin 500 mg for a UTI — how many days?
Uncomplicated UTIs are typically treated for 3–7 days, though the exact duration depends on the specific diagnosis and local guidance — always complete the full course prescribed, even if symptoms improve sooner.
What does the BNF say about cephalexin?
The British National Formulary lists cephalexin's standard adult and child (weight/age-banded) doses, typically split into 2–4 doses a day depending on the indication and severity — see the BNF or BNFc for the current figures for a specific infection.
What are the side effects of cephalexin?
The most common are gastrointestinal — diarrhoea, nausea, and abdominal discomfort. Less common effects include rash and hypersensitivity reactions; as with any cephalosporin, there's a small chance of cross-reactivity in patients with a severe penicillin allergy.
Can cephalexin be used if someone is allergic to penicillin?
It depends on the type of allergy — cross-reactivity between penicillins and cephalosporins is low (roughly 1–3%) but higher with a history of severe/anaphylactic penicillin reactions, where cephalosporins are generally avoided. Confirm with an allergy history review.
How is the child's dose of cephalexin worked out?
It's calculated from the child's weight in kg, multiplied by the recommended mg/kg/day for the infection being treated, then divided across the prescribed number of daily doses.
Does cephalexin treat MRSA infections?
No — cephalexin is not active against methicillin-resistant Staphylococcus aureus (MRSA). A different agent is needed if MRSA is suspected or confirmed.
References
- American Academy of Pediatrics Red Book — Report of the Committee on Infectious Diseases.
- British National Formulary for Children (BNFc) — Cephalexin.
- Stevens DL, et al. Practice Guidelines for Skin and Soft Tissue Infections (IDSA). Clin Infect Dis. 2014.