Cefalexin

Basic Cefalexin information

• INN (international nonproprietary name): Cefalexin.
• Brand names available in the United Kingdom: Keflex; generic cefalexin.
• ATC code: J01DB01 (systemic antibacterials, β‑lactam antibiotics, first‑generation cephalosporins).
• Forms and dosages: capsules/tablets 250 mg, 500 mg; oral suspension 125 mg/5 ml, 250 mg/5 ml (powder for reconstitution).
• Manufacturers in the United Kingdom: various generic manufacturers.
• Registration status in the United Kingdom: Prescription-only medicine (POM), listed in the BNF.
• OTC / Rx classification: Prescription-only medicine (POM).

Cefalexin is a first‑generation cephalosporin antibiotic. Check the batch number, expiry date and manufacturer name on the pack, and use only products supplied for human use.

Product forms and strengths

Choosing the correct formulation for age and indication

Cefalexin is supplied as tablets or capsules in 250 mg and 500 mg strengths and as oral suspension in 125 mg/5 ml and 250 mg/5 ml strengths.

Tablets or capsules are generally recommended for adults who can swallow solid dosage forms.

Reconstituted oral suspension is preferred for children to allow weight‑based dosing and accurate measurement with a syringe.

Pros of tablets/capsules: convenient dosing, stable at room temperature and suitable for adults; cons: unsuitable for children who cannot swallow.

Pros of suspension: flexible dosing for children by weight and easier to swallow; cons: requires refrigeration after reconstitution and shorter shelf life.

A pharmacist can confirm the correct formulation, check dosing by weight, and advise on reconstitution and measuring equipment.

Approved indications and everyday clinical uses

Common infections treated in primary care

Cefalexin is commonly used in primary care for uncomplicated skin and soft tissue infections, streptococcal pharyngitis, community respiratory infections and uncomplicated urinary tract infections.

For respiratory infections and skin infections, standard adult dosing is typically 250–500 mg every six hours.

For uncomplicated urinary tract infection in adults, usual regimens also fall in the 250–500 mg every six hours band.

For streptococcal pharyngitis a common adult regimen is 500 mg every 12 hours for 10 days to ensure eradication.

Typical treatment durations range from seven to fourteen days depending on the condition and clinical response.

When symptoms worsen, fever persists or red flags appear such as spreading redness or difficulty breathing, patients should seek prompt review.

Local guideline variation and prescriber judgement may alter dosages and durations, so always follow the instructions on the prescription and the prescriber’s advice.

Dosing guidance, weight‑based paediatric dosing and adjustments

How to calculate paediatric doses and renal adjustments

Children are dosed by weight, with recommended ranges approximately 25–50 mg/kg/day divided into suitable doses for the formulation used.

Use an oral suspension such as 125 mg/5 ml or 250 mg/5 ml and a measuring syringe for accurate administration.

Step‑by‑step for parents: weigh the child, multiply weight in kg by the chosen mg/kg dose to get total daily mg, then divide by the number of daily doses to get per‑dose mg, and convert to ml using the suspension concentration.

For example, a 20 kg child at 30 mg/kg/day requires 600 mg daily; given as 200 mg three times a day using a 125 mg/5 ml suspension equals 8 ml per dose.

For adults with renal impairment, dose reduction or extended dosing intervals are required and the product SPC should be consulted for creatinine clearance guidance.

Elderly patients should be monitored for renal function since standard adult doses are used but renal dosing adjustments may be necessary.

Prescriber discretion applies in severe infections and in high body mass index cases where specialist dosing guidance may be used.

Contraindications, warnings and pregnancy/breastfeeding notes

Allergy screening and special populations

Absolute contraindications are a known allergy to cefalexin, any cephalosporin or a history of immediate hypersensitivity to β‑lactam antibiotics such as anaphylaxis.

Relative cautions include a history of penicillin allergy, severe renal impairment, and a history of colitis or significant gastrointestinal disease.

Pregnancy and breastfeeding: cefalexin is commonly used when clearly needed, but prescribers and patients should discuss risks and benefits before starting treatment.

Tell the prescriber or pharmacist about previous reactions to penicillins or cephalosporins and any history of angioedema or severe rashes.

Known β‑lactam anaphylaxis, severe renal impairment and recent C. difficile history are clear flags for medical review.

Anyone with a penicillin allergy should discuss cross‑reactivity risk with the prescriber before treatment.

Side effects, adverse reactions and patient counselling points

Frequency, management and when to seek help

Common mild to moderate side effects include gastrointestinal symptoms such as diarrhoea, nausea, vomiting and abdominal pain.

Other common reactions include skin rash, urticaria, headache, dizziness and fatigue.

Less commonly patients may experience vaginal thrush or oral candidiasis after a course of antibiotics.

There is a risk of Clostridioides difficile‑associated diarrhoea with any systemic antibiotic, including cefalexin.

For mild, tolerable symptoms such as mild diarrhoea or nausea, simple measures help: take with food, maintain hydration and continue the course unless symptoms are severe.

Stop the antibiotic and seek urgent medical help if there are signs of anaphylaxis, severe rash, swelling, difficulty breathing, severe abdominal pain or bloody diarrhoea.

Drug interactions and co‑prescribing considerations

Notable interactions and dose‑modifying combinations

Monitor warfarin INR if cefalexin is prescribed concurrently, as antibiotic therapy can change INR control and monitoring may be required.

Concurrent use with other nephrotoxic agents warrants caution because of combined renal impact and a need for renal monitoring.

There is a theoretical effect on oral contraceptives with some antibiotics, but this is not a proven interaction.

Avoid unnecessary broad‑spectrum antibiotic combinations and consult the prescriber where switching or combining antibiotics is considered.

Storage and reconstitution

How to store capsules, tablets and suspensions safely

Store unopened tablets and capsules at room temperature, typically between 15–30°C, protecting from light and moisture.

Reconstituted oral suspension should be refrigerated between 2–8°C and discarded after 14 days.

Keep medicines in their original packaging and label a reconstituted bottle with the date of reconstitution and the discard date.

Do not use reconstituted suspension after the 14‑day discard date, and return unused medicines to a pharmacy for safe disposal.

Legal status and availability in the UK

Cefalexin is a prescription‑only medicine (POM) in the UK.

In our online pharmacy, cefalexin is available without a prescription, with discreet delivery to the United Kingdom in 5‑14 days.

Alternatives, clinical comparators and stewardship considerations

When to pick a different antibiotic

Alternatives to cefalexin include other first‑generation cephalosporins such as cefadroxil and penicillins such as amoxicillin.

For patients with penicillin allergy macrolides such as azithromycin or clarithromycin are common alternatives, subject to clinical judgement.

When choosing an antibiotic aim for the narrowest effective agent based on likely pathogens and local resistance patterns to support stewardship.

If there is no clinical improvement within the expected timeframe, or if the clinical picture worsens, review therapy promptly and consider culture or alternative agents.

Good stewardship means confirming the indication, verifying duration, reassessing response and avoiding unnecessary broad‑spectrum combinations.

FAQs, aftercare, missed doses and emergency advice

Practical patient guidance, follow‑up and red flags

Missed dose advice: take a missed dose as soon as remembered but skip it if it is almost time for the next dose; do not double up.

Overdose symptoms may include nausea, vomiting and diarrhoea; seek emergency care for severe symptoms.

Expected symptom timelines: many patients notice improvement within 48–72 hours but finish the prescribed course unless advised otherwise.

Red flags prompting immediate review include signs of anaphylaxis, severe diarrhoea with blood, persistent high fever and rapidly spreading skin infection.

Disposal advice: return unused or expired medicines to a pharmacy for safe disposal where facilities exist.

Delivery across the United Kingdom