Cefalexin (Cephalexin)
Is It Right for You?
A complete guide to Cefalexin (Cephalexin) — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Cefalexin?
Cefalexin (also spelled cephalexin) is a first-generation cephalosporin antibiotic used to treat a range of bacterial infections affecting the skin, urinary tract, respiratory system, and other tissues. It is one of the most commonly prescribed oral antibiotics in the UK and is valued for its broad spectrum of antibacterial activity, good oral bioavailability, and well-established safety profile.
Structurally related to penicillin, cefalexin belongs to the beta-lactam family of antibiotics. It acts by interfering with bacterial cell wall synthesis, making it effective against many gram-positive organisms and some gram-negative bacteria. It is a bactericidal antibiotic — meaning it actively kills bacteria rather than simply inhibiting their growth.
Cefalexin is frequently used as an alternative to penicillin in patients who report a penicillin allergy, though clinical evaluation of the nature of the allergy is important, as a small degree of cross-reactivity between penicillins and cephalosporins exists. It is available in capsule and oral suspension forms and is suitable for adults and children.
Cefalexin is a prescription-only medication in the UK. A licensed clinician must assess your symptoms and medical history before it can be prescribed — which can now be done quickly and conveniently through a telehealth consultation.
What Conditions Is Cefalexin Used For?
Cefalexin is prescribed for a variety of bacterial infections:
including uncomplicated cystitis, often as a second-line option.
cellulitis, impetigo, infected wounds, and folliculitis caused by susceptible organisms.
tonsillitis and pharyngitis caused by Streptococcus pyogenes, particularly where penicillin is unsuitable.
including bronchitis and pneumonia caused by susceptible organisms.
as part of treatment regimens for osteomyelitis and septic arthritis.
in patients with a history of rheumatic fever who are unable to take penicillin.
Cefalexin is an antibiotic and has no effect on viruses. It will not treat the common cold, influenza, COVID-19, or other viral illnesses. Using antibiotics for viral infections contributes to antibiotic resistance and should be avoided.
How Does Cefalexin Work?
Cefalexin is a beta-lactam antibiotic that exerts its bactericidal effect by inhibiting bacterial cell wall synthesis. It binds to penicillin-binding proteins (PBPs) — enzymes found on the bacterial cell membrane that are responsible for cross-linking the peptidoglycan strands which give the bacterial cell wall its strength and rigidity.
By inhibiting PBPs, cefalexin prevents the formation of a structurally sound cell wall. As bacteria continue to grow and divide without being able to complete cell wall synthesis, they become osmotically fragile and eventually lyse (burst), leading to bacterial death.
Cefalexin is most effective against gram-positive organisms — including Staphylococcus aureus and Streptococcus species — which have thick peptidoglycan cell walls. It also has activity against some gram-negative bacteria such as Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae, making it useful for urinary tract infections.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Cefalexin can be taken with or without food. Taking it with food may help reduce nausea or gastrointestinal discomfort in sensitive individuals.
Complete the full course even if symptoms improve after a few days. Stopping early risks allowing bacteria to survive, potentially causing a relapse or contributing to antibiotic resistance.
Oral suspension should be refrigerated after reconstitution and discarded after 14 days.
Take at evenly spaced intervals throughout the day to maintain consistent antibiotic blood levels — for example, a four-times-daily regimen should be spread approximately every 6 hours.
Inform your clinician of any penicillin allergy and the nature of the reaction before taking cefalexin, so they can assess whether it is safe for you.
Side Effects of Cefalexin
Cefalexin is generally well tolerated. Most side effects are mild and gastrointestinal in nature.
- Nausea
- Diarrhoea
- Abdominal pain
- Indigestion
- Skin rash
- Headache
- Dizziness
- Oral or vaginal thrush (candidiasis)
- Severe allergic reaction (anaphylaxis) — hives, facial swelling, difficulty breathing, collapse; rare but requires emergency treatment
- C. difficile-associated diarrhoea — persistent, watery, or bloody diarrhoea, cramping, and fever during or after treatment
- Haemolytic anaemia — rare; caused by immune-mediated destruction of red blood cells
- Severe skin reactions — Stevens-Johnson Syndrome or Toxic Epidermal Necrolysis; very rare but serious
- Renal toxicity — uncommon at standard doses; risk increased in patients with pre-existing kidney disease or those taking other nephrotoxic drugs
Cefalexin and penicillins share a beta-lactam ring structure. Cross-reactivity is estimated at approximately 1–2%. Patients with a history of immediate or severe allergic reactions to penicillin (such as anaphylaxis, angioedema, or severe urticaria) should generally avoid cephalosporins. Patients with a minor or delayed penicillin reaction may be able to use cefalexin with caution after clinical assessment. Always inform your clinician of the exact nature of your penicillin allergy.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting cefalexin.
Cefalexin has been shown to increase metformin absorption and plasma concentrations. Blood glucose monitoring may be advisable in diabetic patients during co-administration.
Reduces renal excretion of cefalexin, increasing its blood levels and half-life. This interaction may be used deliberately in some clinical settings but should be noted.
Concurrent use may increase the risk of renal toxicity, particularly in patients with pre-existing kidney impairment.
As with other antibiotics, there is a theoretical concern about reduced contraceptive efficacy. Use additional contraceptive precautions during treatment.
Important Warnings
Although rare, severe hypersensitivity reactions including anaphylaxis can occur with cefalexin. Patients with a history of severe penicillin allergy are at higher risk. Seek emergency care immediately if you develop throat swelling, difficulty breathing, or collapse after taking cefalexin.
Like all antibiotics, cefalexin disrupts the gut microbiome and can predispose to Clostridioides difficile overgrowth. If you develop persistent, watery, or bloody diarrhoea during or after treatment, contact a clinician promptly. Do not use anti-diarrhoeal medications without medical advice in this context.
Cefalexin is primarily excreted by the kidneys. In patients with significant renal impairment, dose adjustments may be necessary to prevent drug accumulation and toxicity. Inform your clinician if you have known kidney disease.
Cefalexin is generally considered safe during pregnancy and is one of the preferred antibiotics for treating infections such as UTIs in pregnant women. It is excreted in breast milk in small quantities; short-term use is generally considered compatible with breastfeeding. Always inform your clinician if you are pregnant or breastfeeding.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Cefalexin FAQs
It depends on the nature of your penicillin allergy. Cefalexin shares structural similarities with penicillin and a small degree of cross-reactivity (approximately 1–2%) exists. If your penicillin reaction was mild (e.g. a rash occurring days after the drug), cefalexin may be used cautiously after clinical assessment. If you have had a severe immediate reaction such as anaphylaxis to penicillin, cefalexin should generally be avoided and an entirely unrelated antibiotic class selected. Always discuss your allergy history in detail with your clinician.
Cefalexin is rapidly absorbed after an oral dose, with peak blood levels reached within 1 hour. Most patients notice an improvement in symptoms within 24 to 48 hours. However, completing the full prescribed course is important even if you feel better, to ensure the infection is fully cleared.
Yes. Cefalexin is commonly used to treat uncomplicated urinary tract infections, particularly as a second-line option or when other agents are unsuitable. It is generally effective against common urinary pathogens such as E. coli, Klebsiella, and Proteus species.
Cefalexin is one of the antibiotics most commonly prescribed in pregnancy and is considered safe for use throughout all trimesters, including for the treatment of UTIs, which are common in pregnancy. Always inform your clinician that you are pregnant so they can make the most appropriate prescribing decision.
Yes. Cefalexin is frequently used in children and is available in oral suspension form, making it suitable for younger patients. Paediatric doses are calculated based on body weight and should be prescribed by a clinician.
Both are oral antibiotics effective against similar organisms, but they belong to different classes — cefalexin is a cephalosporin and amoxicillin is a penicillin. Amoxicillin is often preferred as first-line treatment for throat infections and respiratory infections, while cefalexin is a commonly used alternative in patients with penicillin allergy and is frequently used for skin and urinary infections.