Cephalexin typically starts working within the first 24 to 48 hours, though you may not notice a visible difference in your skin that quickly. Most people see a noticeable reduction in redness, swelling, and pain within 2 to 3 days of starting the medication. If you don’t feel any improvement by day 3, that’s the point to contact your prescriber.
What Improvement Looks Like Day by Day
Cephalexin begins killing bacteria shortly after your first dose, but the visible signs of infection don’t clear immediately. The drug works by weakening the structural walls of bacteria, causing them to break apart. That process starts right away, but your body still needs time to clean up the damage the infection has already caused.
In the first 24 hours, pain may start to ease slightly, but redness and swelling often remain unchanged or even look a little worse before they improve. By 48 to 72 hours, you should notice the redness starting to shrink rather than spread, and the area should feel less warm and tender. The NHS advises contacting your doctor if you don’t feel better after 2 to 3 days, or if you feel worse at any point during treatment.
Full resolution takes longer than those first few days. Redness and firmness in the skin can linger for a week or more even when the antibiotic is working well. The key signal to watch is the direction of change: improving slowly is normal, staying the same or worsening after 48 hours is not.
How Long the Full Course Lasts
The standard course for skin infections is 5 to 14 days, depending on severity. For uncomplicated cellulitis, the Infectious Diseases Society of America recommends 5 days of antibiotics, with the option to extend if the infection hasn’t improved by then. A clinical trial of 121 patients with uncomplicated cellulitis found identical cure rates (98%) whether patients took antibiotics for 5 days or 10 days.
For more severe or deeper infections, courses of 7 to 14 days are common. Your prescriber will determine the length based on how the infection looks and responds. Finishing the entire prescribed course matters even after your skin looks better, because bacteria can survive in smaller numbers and regrow if treatment stops too early.
What Cephalexin Works Best Against
Cephalexin is a first-generation cephalosporin, which means it’s particularly effective against the two bacteria responsible for most skin infections: staph (Staphylococcus aureus) and strep (Streptococcus pyogenes). These are the organisms behind cellulitis, impetigo, and many wound infections.
One important limitation: cephalexin does not reliably cover MRSA (methicillin-resistant Staphylococcus aureus). If your infection involves an abscess that was drained, this may matter less than you’d think. In a randomized trial, patients with skin abscesses caused by community MRSA strains had a 90.5% cure rate with placebo after drainage, compared to 84.1% with cephalexin. The takeaway is that for drained abscesses, the drainage itself does most of the work. But for spreading skin infections without an abscess, MRSA resistance can be a real problem. If your infection isn’t improving on cephalexin, MRSA is one of the first things your provider will consider.
Signs the Antibiotic Isn’t Working
A helpful trick is to draw a line around the edge of the redness with a pen when you start treatment. This gives you a concrete reference point. If the redness crosses that line and keeps expanding after 48 hours on cephalexin, that’s a clear sign something needs to change.
Other red flags include:
- Persistent or new fever after 2 or more days of treatment
- Increasing pain rather than gradual improvement
- Blistering or darkening skin over the infected area
- Red streaks spreading outward from the infection site
Any of these can indicate that the bacteria are resistant to cephalexin, that the infection is deeper than initially thought, or that an abscess has formed and needs drainage. Rapidly spreading redness or worsening symptoms warrant urgent evaluation rather than waiting for a scheduled follow-up.
Why Some People Heal More Slowly
Not everyone responds to cephalexin on the same timeline. Diabetes is one of the most common reasons for slower healing. High blood sugar causes thickening in the walls of small blood vessels, which reduces blood flow to the infected area and makes it harder for both your immune cells and the antibiotic to reach the site in full concentration. Diabetes also blunts the typical inflammatory response, which can make it difficult to tell whether an infection is truly improving or just appearing quiet on the surface.
Other factors that slow healing include poor circulation from peripheral artery disease, immune suppression from medications like steroids or chemotherapy, obesity (which affects drug distribution and blood flow to skin), and infections located on the lower legs or feet where circulation is naturally weaker. If any of these apply to you, a slower improvement curve doesn’t automatically mean the antibiotic is failing, but it does mean closer follow-up is worthwhile.
Common Side Effects During Treatment
Stomach-related side effects are the most frequent complaint with cephalexin. Nausea, diarrhea, and mild abdominal discomfort affect a meaningful number of people, particularly at higher doses. Taking the medication with food can reduce stomach upset without affecting how well the drug is absorbed.
These digestive symptoms can be confusing if you’re also monitoring for signs of worsening infection. Feeling generally unwell from an upset stomach is different from the fever, chills, and spreading redness that signal treatment failure. If your skin is clearly improving but your stomach is unhappy, the antibiotic is likely doing its job.