Cefalexin (cephalexin)
A first-generation oral cephalosporin used for UTIs (including in pregnancy), mild pyelonephritis and skin infections.
On this page
Uses
- Urinary tract infection, including in pregnancy.
- Mild pyelonephritis (NICE first choice where sensitive).
- Skin and soft tissue infections (alternative to flucloxacillin).
Adult dose
| Indication | Dose |
|---|---|
| UTI (including pregnancy) | 500 mg twice daily for 7 days |
| Pyelonephritis | 500 mg 2–3 times daily (up to 1–1.5 g 3–4 times daily in severe infection) for 7–10 days |
| Skin infection | 500 mg 3–4 times daily |
Children's dose
25 mg/kg twice daily (up to 50 mg/kg in severe infection; max 1 g per dose).
How it works
Cefalexin is a beta-lactam that binds penicillin-binding proteins and blocks bacterial cell wall synthesis, killing dividing bacteria. As a first-generation cephalosporin it is strongest against gram-positive cocci (streptococci and meticillin-sensitive Staph. aureus) with useful activity against many E. coli, Klebsiella and Proteus mirabilis. It is excreted unchanged in the urine, so urinary levels are high.
When it is used
- Lower UTI in pregnancy, where nitrofurantoin is unsuitable (for example near term), and pyelonephritis in pregnancy as the oral first choice.
- Acute pyelonephritis in non-pregnant adults where the patient is well enough for oral treatment.
- Skin and soft tissue infection when flucloxacillin is not tolerated, per local policy.
- Strep throat as an alternative to penicillin in non-severe allergy.
It does not cover enterococci, Pseudomonas, MRSA, anaerobes, atypical organisms or ESBL-producing gram-negatives. It is not for meningitis, sepsis or any infection needing IV treatment.
Cautions
Allergy
- Avoid if there is a history of anaphylaxis or a severe reaction to any beta-lactam.
- Cross-reactivity with penicillin is low overall, but cefalexin shares a side chain with amoxicillin and ampicillin, so reactions to those drugs deserve extra caution.
Kidney and liver
Reduce dose or frequency if creatinine clearance below 30. Use the kidney function calculator to estimate clearance. No adjustment is needed in liver disease.
Side effects
- Common: diarrhoea, nausea, abdominal pain, vaginal thrush.
- Serious but uncommon: C. difficile colitis, severe skin reactions, interstitial nephritis.
- Can give a false-positive Coombs test and false-positive urine glucose with older reduction-based tests.
Interactions
Few important ones. Probenecid raises levels.
Monitoring
- Send a urine culture before starting in pregnancy, pyelonephritis, men, children and recurrent infection. E. coli resistance to cephalosporins is common in Pakistan, so culture results often change management.
- Review at 48 hours. If pyelonephritis is not improving, recheck the culture and consider imaging and IV treatment.
- In pregnancy, send a repeat urine culture after treatment as a test of cure.
Pregnancy and breastfeeding
Considered safe.
Cefalexin is one of the most used antibiotics in pregnancy and is compatible with breastfeeding.
Practical tips
- It can be taken with or without food.
- Capsules are usually 250 mg and 500 mg; suspensions come in more than one strength (for example 125 mg/5 mL and 250 mg/5 mL). Check the strength before converting a child's dose to mL.
- Cephradine, another first-generation oral cephalosporin, is widely used in Pakistan; it is not interchangeable mg for mg without checking its own dose.
- Do not use cefalexin for a UTI already shown to be ESBL-positive.
- Common errors: twice-daily dosing for cellulitis (it needs 3 to 4 doses a day) and no culture before treating pyelonephritis.
See uncomplicated UTI, pyelonephritis and cellulitis. Alternatives include nitrofurantoin, fosfomycin and flucloxacillin. For FCPS, revise antibacterial drugs and antimicrobials in pregnancy.
Frequently asked questions
Is cefalexin safe in pregnancy?
Yes. It is commonly used for UTI in pregnancy.
What is the cefalexin dose for pyelonephritis?
500 mg two or three times a day for 7–10 days, higher in severe infection.
Can I give cefalexin to a patient with penicillin allergy?
Avoid it after anaphylaxis or a severe reaction to penicillin. After a mild rash, many clinicians use it with care, but remember it shares a side chain with amoxicillin.
Does cefalexin cover Pseudomonas or enterococcus?
No. It has no useful activity against Pseudomonas or enterococci, so it should not be used when these are likely.
Used in these guides
Sources
- Drugs.com: Cephalexin dosage
- NICE NG111: Pyelonephritis (acute): antimicrobial prescribing
- NICE NG109: Urinary tract infection (lower): antimicrobial prescribing
Doses on this page are typical doses from the sources above. Always confirm with a current drug reference, the product leaflet and your hospital protocol, especially in children, pregnancy and kidney or liver disease.
For FCPS Part 1
Revising for FCPS Part 1? Cefalexin is covered in our textbook chapter Antimicrobials in pregnancy, with practice MCQs.