Cephalexin for a tooth infection is typically prescribed for 3 to 7 days, at a dose of 500 mg taken four times daily. The exact duration depends on how quickly your symptoms improve. Current American Dental Association guidelines recommend stopping the antibiotic 24 hours after your symptoms resolve, even if that means finishing sooner than 7 days.
The 3-to-7-Day Window
The ADA’s clinical practice guideline for managing dental pain and intraoral swelling specifies oral cephalexin at 500 mg, four times per day, for 3 to 7 days. That’s a wider range than many people expect, and it reflects a shift in how dentists approach antibiotic courses. Rather than a fixed number of days, the guideline ties the endpoint to your actual recovery: your dentist should check in within 3 days (by phone or in person), and you should stop taking the antibiotic 24 hours after symptoms like pain, swelling, and fever have cleared up.
This means some people finish in as few as 3 or 4 days, while others need the full week. Taking four doses a day, spaced roughly 6 hours apart, keeps the drug at effective levels in your system. Missing doses or stopping too early can allow bacteria to rebound, while taking it longer than necessary offers no extra benefit and increases the chance of side effects.
When You’ll Start Feeling Better
Most people notice less pain and reduced swelling about 48 to 72 hours after starting cephalexin. That first day or two can feel discouraging because the antibiotic needs time to kill enough bacteria to make a noticeable difference. Cephalexin works by destroying bacterial cell walls, which causes the bacteria to die off gradually rather than all at once.
Over-the-counter pain relievers like ibuprofen can bridge that gap while you wait for the antibiotic to take hold. If you reach the 3-day mark and your symptoms haven’t improved at all, or they’re getting worse, that’s a signal to contact your dentist. The infection may need a different antibiotic, or it may require a procedure like drainage or a root canal to resolve.
Why Cephalexin Instead of Other Antibiotics
Cephalexin is a first-generation cephalosporin, which means it’s closely related to penicillin-type drugs like amoxicillin. It’s effective against several types of bacteria commonly found in dental infections, including streptococcal species. Dentists often prescribe it as an alternative when amoxicillin alone isn’t the best fit, or as part of a targeted approach based on the type of infection.
If you have a penicillin allergy, the situation gets a bit more nuanced. Because cephalexin is a first-generation cephalosporin, cross-reactivity with penicillin allergies falls in the 1% to 8% range. That’s low but not negligible, especially if your penicillin allergy involved hives or anaphylaxis. For comparison, newer-generation cephalosporins have a cross-reactivity rate below 1%. Anaphylaxis from a cephalosporin in someone with a reported penicillin allergy is estimated at roughly 1 in 52,000 cases. Still, make sure your dentist knows your full allergy history before you start.
Common Side Effects During Treatment
The most frequent complaints with short-term cephalexin use are gastrointestinal: nausea, diarrhea, stomach discomfort, and occasionally vomiting. Taking the capsule with food can reduce stomach upset. Some people also develop a mild skin rash. These side effects are usually manageable and resolve once you finish the course.
Diarrhea is worth paying attention to. Mild loose stools are common with any antibiotic, but frequent watery diarrhea, especially with cramping or blood, could signal a more serious gut infection that needs medical attention. Eating probiotic-rich foods like yogurt during your course may help maintain healthy gut bacteria, though results vary from person to person.
The Antibiotic Alone Won’t Fix the Problem
Cephalexin controls the bacterial infection, but it doesn’t treat the underlying dental issue. A tooth infection typically starts from deep decay, a crack in the tooth, or advanced gum disease. Once the antibiotic course brings the acute infection under control, you’ll still need a dental procedure (a filling, root canal, extraction, or drainage) to prevent it from coming back. Skipping that follow-up is one of the most common reasons people end up with repeat infections.
Signs the Infection Is Spreading
Most tooth infections respond well to antibiotics and dental treatment, but a small number can escalate quickly. A fever above 100.4°F suggests the infection has moved beyond the tooth. Swelling that extends to your eye, making it hard to open, or down into your neck is a red flag that needs emergency care.
The most dangerous complication is swelling in the soft tissues of the throat and neck that restricts your airway. This condition can become life-threatening within hours. Difficulty swallowing, difficulty breathing, or trouble opening your mouth are all reasons to go to the emergency room immediately, even if you’re already taking an antibiotic. Rapid facial swelling that’s visibly worsening over the course of hours, rather than days, also warrants an ER visit. These scenarios are uncommon, but recognizing them matters.