Cephalexin stops bacteria building their cell wall. Without a wall, the bacterium cannot hold its shape against its own internal pressure and bursts. Human cells have no cell wall, which is why the drug can kill bacteria efficiently without doing much to you.
In dermatology it earns its place through coverage rather than power. It reliably hits Staphylococcus aureus of the ordinary, methicillin-sensitive kind, and Streptococcus pyogenes — between them responsible for most cellulitis, most impetigo that needs a pill, most infected cuts, boils and infected nail folds. It does not cover MRSA, and it does not cover the mixed bacteria in a human or animal bite, which is why those get different prescriptions.
Alcohol does not interact with cephalexin. There is no reaction, unlike with metronidazole. Heavy drinking will add to any stomach upset, but a drink is fine.
Metformin levels can rise when taken with cephalexin. Tell your prescriber if you take it.
Probenecid, used for gout, raises cephalexin levels by slowing its clearance.
Warfarin may need closer monitoring, as with most antibiotics.
Zinc supplements taken at the same time can reduce absorption. Space them a couple of hours apart.
Probiotics or live yoghurt are worth taking during the course if you are prone to thrush or diarrhoea, spaced a couple of hours from the doses. Evidence for this is modest, but the downside is nil.
No interaction with sun, and no reason to change anything about sun protection.
A standard seven-day course in a healthy person needs no blood tests.
What is monitored is the infection itself. The expectation is that redness stops spreading within 24 to 48 hours and that pain and fever settle. A marked edge makes this easy to judge. Failure to improve in that window is the trigger to re-examine rather than to extend the course.
A wound swab or culture is worth taking when there is pus to sample, when infections keep returning, or when a first course has failed — it identifies whether MRSA is involved and which antibiotics will work.
Kidney function matters for dosing. In known kidney disease, or in older adults on repeated courses, the dose is adjusted and kidney function may be checked.
Fever and feeling unwell usually improve within 24 to 48 hours.
Redness stops spreading in the same window. It does not immediately shrink — in cellulitis the red area often looks slightly worse on day one as the killed bacteria release their contents, which is expected and not a treatment failure.
By day three to four the area should be clearly smaller, less tender and less warm.
By the end of the course it should be settled, though skin can stay pink or discoloured for weeks afterwards. On brown and Black skin, cellulitis and impetigo often leave darker marks that take one to three months to fade — those are pigment left by the inflammation, not lingering infection.
No improvement by 48 to 72 hours is the signal to go back. The usual explanations are MRSA, an abscess that needs draining rather than antibiotics, or something that was never an infection — stasis dermatitis on the lower legs is very commonly treated as cellulitis and does not respond to antibiotics because there is nothing to kill.
When the course finishes, that is the end of it — no taper, no rebound.
Stopping early is the real risk. Skin infections improve visually well ahead of the bacteria being cleared, and an interrupted course is one of the more common reasons cellulitis recurs a week or two later.
Recurrence despite a completed course points somewhere else. Repeated cellulitis in the same leg usually means an entry point that has not been dealt with — most often athlete's foot cracking the skin between the toes, or swelling from poor circulation. Repeated boils usually mean staph carried in the nose. Treating the infection over and over without addressing the entry point is a familiar and avoidable loop.
After any antibiotic course, gut bacteria take a few weeks to recover, and mild looseness or thrush during that window is common.