The first-choice antibiotic for cellulitis depends on whether the infection involves pus. For typical cellulitis without an abscess or drainage, the standard first-line options are cephalexin, dicloxacillin, amoxicillin, or penicillin, all taken by mouth. These target the streptococcal bacteria responsible for most non-purulent cellulitis cases. A standard course lasts five days, though it may be extended if the infection hasn’t improved.
Non-Purulent Cellulitis: The Most Common Type
Most cellulitis presents as a spreading area of redness, warmth, and swelling without a visible pocket of pus. The primary culprit is group A streptococcus, which means the antibiotic only needs to reliably kill strep bacteria. The Infectious Diseases Society of America recommends cephalexin, penicillin, amoxicillin, amoxicillin-clavulanate, dicloxacillin, or clindamycin as suitable oral options for mild cases.
In practice, cephalexin is the most commonly prescribed of these because it covers both streptococcus and methicillin-susceptible staph, giving a slightly broader safety net. It’s typically taken four times a day (every six hours) or three times a day at a higher dose. Children’s dosing is weight-based, usually around 17 mg/kg per dose three times daily, with cephalexin also serving as the pediatric first-line choice.
When Pus or an Abscess Changes the Choice
If cellulitis surrounds a draining wound or abscess, the likely cause shifts from streptococcus to staphylococcus aureus, including community-acquired MRSA. Standard antibiotics like cephalexin don’t reliably cover MRSA, so the first-line picks change to clindamycin or trimethoprim-sulfamethoxazole (often called TMP-SMX or Bactrim).
A decision analysis comparing these options found that clindamycin has the highest probability of being effective against the likely bacteria (about 95%), followed by TMP-SMX at roughly 89%. Cephalexin only comes out on top when the local MRSA rate is below 10%, which is uncommon in most U.S. communities today. Your prescriber will often choose between clindamycin and TMP-SMX based on local resistance patterns and your tolerance for side effects, since clindamycin is more likely to cause digestive issues.
What If You’re Allergic to Penicillin?
Cephalexin is technically a different class of antibiotic (a cephalosporin), and most people with a mild penicillin allergy, such as a rash years ago, can take it safely. If your reaction to penicillin was severe, such as throat swelling or anaphylaxis, clindamycin or erythromycin are the usual alternatives. Resistance to erythromycin has been increasing in some areas, so your provider may lean toward clindamycin. Doxycycline (100 mg twice daily) is another option that avoids the penicillin family entirely.
Signs That Require Stronger Treatment
Mild cellulitis, meaning localized redness without fever or other body-wide symptoms, can be treated entirely with oral antibiotics at home. When the infection triggers systemic signs like fever, rapid heart rate, or confusion, intravenous antibiotics become necessary. Clinicians classify cellulitis severity on a scale from mild (class I) to life-threatening (class IV), and anything at class III or above, where you’re showing signs of sepsis or have poorly controlled underlying conditions, typically means hospital admission.
Certain risk factors also push treatment toward broader coverage. If you have a history of MRSA, use injection drugs, or developed cellulitis after a penetrating wound, guidelines recommend an antibiotic effective against both MRSA and streptococcus from the start, even if there’s no visible pus.
How Long Treatment Takes to Work
You should start feeling better within two to three days of starting antibiotics. Visible skin improvement, where the redness begins to shrink and the swelling softens, often takes about 72 hours. By around day 10, studies have found that swelling typically decreases by about 50% and the affected area shrinks by roughly 55%.
The CDC recommends a five-day course for most uncomplicated cellulitis. If you’re not seeing improvement by day three to five, that doesn’t necessarily mean the infection is dangerous, but it does mean the antibiotic choice may need to change. Worsening redness, increasing pain, or the infection spreading to new areas are the clearest signals that your current treatment isn’t working. For patients who started on IV antibiotics in the hospital, a switch to oral medication is usually possible after one to four days once fever resolves and the redness starts fading.
Practical Tips During Treatment
Elevating the affected limb helps reduce swelling and can speed recovery noticeably. Drawing a line around the edge of the redness with a pen gives you a simple way to track whether the infection is spreading or shrinking. Finish the full antibiotic course even if you feel better early, since stopping short increases the risk of the infection returning. If you develop diarrhea, particularly with clindamycin, contact your provider rather than stopping the medication on your own.