Cephalexin

is prescribed to treat:

• Bacterial infections including skin, respiratory, and urinary tract infections • Various related conditions in the antibiotics category • Associated symptoms and complications

It is an important medication that helps manage these conditions effectively.

Cephalexin (Keflex) is a first-generation cephalosporin used for skin and soft-tissue infection, streptococcal pharyngitis, uncomplicated urinary tract infection, and bone and joint infection. It is a workhorse outpatient antibiotic — narrow enough to be a responsible choice, effective against the organisms that cause most community skin infections, and inexpensive.

Mechanism of Action

Cephalexin is a beta-lactam. It binds penicillin-binding proteins and blocks the transpeptidase cross-linking that gives the bacterial cell wall its structural integrity, so the organism cannot withstand internal osmotic pressure and lyses. Human cells have no peptidoglycan wall, which is why beta-lactams are so well tolerated.

As a first-generation cephalosporin, its spectrum is weighted toward Gram-positive organisms — particularly methicillin-sensitive Staphylococcus aureus and Streptococcus species — with modest Gram-negative coverage including many E. coli, Klebsiella, and Proteus. Later cephalosporin generations trade Gram-positive potency for broader Gram-negative reach, so cephalexin is actually better than a third-generation agent for a straightforward staphylococcal skin infection.

It does not cover MRSA. This is the single most important limitation in practice, since community-acquired MRSA is common in skin and soft-tissue infection. Where MRSA is suspected — a purulent abscess, prior MRSA, or failure on cephalexin — doxycycline, trimethoprim-sulfamethoxazole, or clindamycin is needed instead or in addition.

Like other beta-lactams, cephalexin is time-dependent: efficacy tracks the proportion of the dosing interval during which drug concentration exceeds the organism's minimum inhibitory concentration. Its short half-life is why it is dosed several times daily, and why spreading doses matters more than the daily total.

Clinical Use

Cephalexin is first-line for non-purulent cellulitis, where streptococci predominate and MRSA is less likely. For a purulent abscess, drainage is the primary treatment and MRSA coverage is usually appropriate — a distinction that determines antibiotic choice more reliably than the appearance of the surrounding skin does.

It is an alternative for streptococcal pharyngitis, useful in patients with a non-severe penicillin allergy, and it treats uncomplicated urinary infection, including in pregnancy where its safety record is strong.

The penicillin allergy question comes up constantly here. Cross-reactivity between penicillins and cephalosporins is far lower than the historically quoted ten percent, and modern estimates put it around one to two percent, concentrated among agents sharing similar side chains. A patient with a history of a mild childhood rash on amoxicillin can generally receive cephalexin safely. A patient with documented anaphylaxis to a penicillin should not, without allergy assessment. Distinguishing these two histories is worth the two minutes it takes, because the alternative is a broader, more toxic antibiotic for no reason. The antibiotic resistance article covers why narrow-spectrum choices matter, and the CDC antibiotic prescribing resource sets out the stewardship rationale.

Monitoring and Follow-Up

Short courses in healthy patients require no laboratory monitoring. Prolonged therapy — for bone or joint infection, for instance — warrants periodic blood count, kidney function, and liver enzyme checks.

Clinical response is the key follow-up. In cellulitis, improvement in erythema, warmth, and pain within 48 to 72 hours is expected. Failure to improve suggests MRSA, an undrained abscess, a resistant organism, or a non-infectious mimic — venous stasis dermatitis is frequently mistaken for bilateral cellulitis, and bilateral lower leg redness is far more often stasis than infection. Escalating the antibiotic in that situation treats the wrong problem.

Cephalexin is renally cleared and requires dose adjustment in significant kidney impairment. It has few drug interactions, which is one of its practical merits. Severe or persistent watery diarrhoea during or after a course raises the possibility of C. difficile. The MedlinePlus cephalexin entry covers prescribing detail.

Course length and dosing frequency are both worth getting right. Cephalexin's short half-life means it is dosed three or four times daily, and adherence falls measurably with each additional daily dose — a patient taking it twice a day because four is impractical is under-treating without realising it. Where frequency is genuinely unworkable, a different agent with less demanding dosing is a better answer than a schedule the patient cannot follow. Course length for uncomplicated cellulitis has also shortened, with five to six days now generally sufficient in patients who are improving, rather than the ten to fourteen days once routinely prescribed.

Special Populations

In pregnancy cephalexin is well established and is a common choice for urinary infection. It is compatible with breastfeeding.

In older adults it is generally well tolerated, with dose adjustment for kidney function the main consideration. In children it is widely used with weight-based dosing. Patients with documented severe IgE-mediated penicillin allergy — anaphylaxis, angioedema, bronchospasm — should avoid cephalosporins without allergy evaluation, while those with a non-severe delayed rash history can usually receive them.

One distinction worth drawing for patients is between an antibiotic that is failing and an infection that is simply slow. Cellulitis often looks worse for the first 24 to 48 hours as inflammation continues even once bacterial growth is controlled, and the redness may extend slightly before it recedes. Marking the border of the erythema with a pen at the start gives an objective reference point and prevents both premature escalation and dangerous complacency.

When to Contact Your Doctor

Seek emergency care for difficulty breathing, swelling of the face, lips, or tongue, or a rapidly spreading hive-like rash. Report any rash so it can be characterised rather than becoming a permanent allergy label by default.

Report severe or persistent watery diarrhoea with abdominal cramping, during or in the weeks after a course. Report failure to improve within two to three days, spreading redness, fever, or increasing pain — cellulitis that is worsening on treatment needs prompt reassessment rather than a longer course. Any blistering or peeling rash needs immediate evaluation.

To discuss a skin infection that is not improving, review an antibiotic allergy label, or address recurrent infection, contact us or schedule a visit.