Penicillin and amoxicillin are the first-line antibiotics for strep throat. They’ve been the standard treatment for decades because Group A Streptococcus, the bacteria behind strep, has never developed resistance to them. Most people start feeling better within one to two days of their first dose, and they become non-contagious within about 12 hours of starting treatment.
Several other antibiotics work well too, especially if you’re allergic to penicillin. Here’s what your doctor is choosing from and why.
Penicillin and Amoxicillin: The Go-To Options
Penicillin V and amoxicillin are recommended as the first choice for strep throat by the CDC. Both work by disrupting the bacteria’s ability to build and maintain its cell wall, which effectively kills it. Group A Strep has shown zero resistance to these drugs, making them remarkably reliable.
Amoxicillin is often preferred for children because it tastes better in liquid form and can be taken just once or twice a day. Adults typically take penicillin V twice daily. Both require a full 10-day course. That duration matters: even though you’ll feel better in a day or two, stopping early gives surviving bacteria a chance to bounce back and can leave you vulnerable to complications.
For people who struggle to complete a 10-day pill regimen, there’s a one-shot option. A single injection of penicillin G eliminates the need to remember daily doses entirely. It’s especially useful for children or anyone with a history of not finishing their prescriptions.
Options If You’re Allergic to Penicillin
Penicillin allergies are one of the most commonly reported drug allergies, so alternatives come up frequently. The options depend on how severe your allergy is.
If your reaction to penicillin was mild (a minor rash, for example), a first-generation cephalosporin like cephalexin or cefadroxil is typically safe. These are closely related to penicillin but different enough that most people with mild penicillin allergies tolerate them without problems. Both are taken for 10 days.
If your allergy is more serious, involving throat swelling, difficulty breathing, or anaphylaxis, cephalosporins are off the table because of the structural overlap. In that case, your doctor will turn to a different class of antibiotic altogether:
- Clindamycin: Taken three times daily for 10 days. It works through a completely different mechanism than penicillin, so there’s no cross-reactivity risk.
- Azithromycin: A 5-day course, which is the shortest regimen available for strep. You take a higher dose on day one, then a lower dose for the remaining four days.
- Clarithromycin: Taken twice daily for 10 days.
There’s an important catch with azithromycin and clarithromycin. Both belong to a class called macrolides, and resistance among strep bacteria to these drugs is climbing. While exact rates vary by region, resistance is common enough that these antibiotics are considered backup options rather than first choices. If your doctor prescribes one, they may want to confirm through a culture that the bacteria will respond to it.
Why the Full Course Matters
Ten days of antibiotics can feel like overkill when your throat stops hurting after 48 hours. But the full course serves two purposes beyond clearing your current infection.
The first is preventing rheumatic fever, a serious inflammatory condition that can damage the heart valves. A meta-analysis published in BMC Cardiovascular Disorders found that antibiotics reduce the risk of rheumatic fever by about 70%. When penicillin specifically was used, that protection jumped to 80%. Rheumatic fever is rare in developed countries precisely because strep gets treated so aggressively.
The second reason is preventing other complications like peritonsillar abscess (a pocket of pus behind the tonsils) and kidney inflammation. Completing the full course gives the antibiotic enough time to eliminate the bacteria entirely, not just knock it down to the point where you feel fine.
What to Expect After Starting Treatment
Most people notice real improvement within one to two days of their first dose. Fever typically breaks first, followed by a gradual easing of throat pain. If you’re not feeling any better after two days, contact your doctor. That could mean the bacteria aren’t responding to the prescribed antibiotic or that something else is going on.
You become non-contagious within 12 hours of your first dose. Schools and daycares use this as their benchmark, allowing kids to return after they’ve been on antibiotics for at least 12 hours and their fever has resolved.
Common Side Effects
The most frequent side effects across all strep antibiotics are digestive: nausea, diarrhea, stomach pain, and occasionally vomiting. These tend to be mild and manageable, especially if you take your medication with food. Amoxicillin can sometimes cause a rash that isn’t a true allergy but looks alarming, particularly in children. This is worth mentioning to your doctor, but it doesn’t always mean you need to switch medications.
Cephalexin shares the same digestive side effects and can also cause dizziness, tiredness, or headache in some people. Clindamycin is more likely than other options to cause diarrhea because it significantly disrupts gut bacteria. With any antibiotic, watch for signs of an allergic reaction: hives, facial swelling, or difficulty breathing. Those require immediate medical attention.
Why Strep Needs an Antibiotic at All
Plenty of sore throats are caused by viruses and clear up on their own. Strep is different. Group A Streptococcus is a bacterial infection, and while your immune system can eventually fight it off without help, the risks of letting it run its course are real. Untreated strep can linger for weeks, during which time you’re contagious and exposed to the possibility of rheumatic fever, kidney problems, or the infection spreading to surrounding tissue.
Antibiotics dramatically shorten that window. They cut your contagious period from weeks down to hours, relieve symptoms faster, and reduce your complication risk by the better part of 70 to 80 percent. For a course of inexpensive, well-tolerated medication, that’s a significant return.