The most common side effects of amoxicillin are diarrhea, nausea, and skin rash, with diarrhea occurring in roughly 1% to 10% of people who take it. Most of these effects are mild and resolve on their own once the course of antibiotics is finished. But some reactions, particularly certain rashes and persistent diarrhea, deserve closer attention because they can signal something more serious.
Digestive Side Effects
Diarrhea, nausea, and stomach pain are the side effects you’re most likely to notice. Observational studies put the rate of diarrhea at around 2% for amoxicillin on its own. When amoxicillin is combined with clavulanic acid (a common pairing sold under various brand names), that rate jumps to about 10%. Nausea and vomiting also occur but weren’t found to be significantly more common than what people experienced on placebo in controlled trials, suggesting they may be partly related to the illness itself rather than the drug alone.
The reason amoxicillin upsets your stomach comes down to what it does to your gut bacteria. Amoxicillin is a broad-spectrum antibiotic, meaning it kills a wide range of bacteria, not just the ones making you sick. From the very first day of treatment, it starts disrupting the balance of microbes in your intestines. Animal research shows that amoxicillin depletes bacteria responsible for producing butyrate, a fatty acid that helps keep your intestinal lining healthy and your digestion running smoothly. With those bacteria suppressed, the gut produces fewer of these protective compounds, which can lead to loose stools and cramping.
The good news is that digestive symptoms are usually transient. They tend to start within the first day or two of treatment and clear up shortly after you finish the course. Taking amoxicillin at the start of a meal or snack can help reduce stomach upset, according to Mayo Clinic guidance. You can take it with or without food since food doesn’t interfere with how well the drug works, but eating something first gives your stomach a buffer.
Skin Rashes: Harmless vs. Allergic
Rashes are one of the most talked-about side effects of amoxicillin, yet a large meta-analysis of placebo-controlled trials found no significant increase in rash rates among people taking the drug compared to those on placebo. That doesn’t mean rashes don’t happen. It means the picture is more complicated than “amoxicillin causes rash.”
The most important thing to understand is that not every rash during amoxicillin is an allergic reaction. Up to 70% of people who take amoxicillin while fighting certain viral infections, particularly mono (Epstein-Barr virus), develop a flat, blotchy rash that looks alarming but is completely harmless. This rash isn’t a drug allergy. It’s an interaction between the virus and the antibiotic that resolves on its own. Many people who get labeled as “penicillin allergic” after this kind of rash aren’t allergic at all. About 10% of U.S. patients have a penicillin allergy on their medical record, but when formally tested by allergists, fewer than 1% turn out to be truly allergic.
A genuine allergic reaction falls into two categories. Immediate reactions happen within 30 to 60 minutes of taking the drug and can include hives, facial swelling, or in rare cases, anaphylaxis involving multiple organ systems. These are driven by a specific immune response and can be life-threatening. Non-immediate reactions appear more than an hour after ingestion, sometimes days into treatment, and typically show up as a widespread bumpy rash or hives that last several days. Most non-immediate reactions are mild and self-resolving, though they still warrant a conversation with your prescriber about whether to continue the antibiotic.
Yeast Infections
Because amoxicillin wipes out many of the bacteria that normally keep yeast in check, it increases the risk of yeast infections (candidiasis). A meta-analysis found that the odds of developing candidiasis were nearly eight times higher during amoxicillin treatment compared to placebo. For every 27 courses of amoxicillin prescribed, roughly one additional case of candidiasis occurred that wouldn’t have happened otherwise.
Vaginal yeast infections are the most commonly reported form. Women who used any antibiotic in the month before a clinic visit were about 75% more likely to have a symptomatic yeast infection than women who hadn’t taken antibiotics, regardless of which antibiotic they used. Oral thrush (white patches in the mouth) can also occur, and infants on amoxicillin-clavulanic acid develop diaper rash at rates as high as 50% in some studies, likely driven by the same fungal overgrowth. These infections are treatable and usually clear up quickly, but they’re worth being aware of, especially if you’ve had yeast infections before.
Serious but Rare Reactions
Most people finish a course of amoxicillin without any problems beyond mild stomach upset, but a few serious reactions can occur. One to watch for is severe, watery, or bloody diarrhea, sometimes accompanied by fever and stomach cramps. This can signal an overgrowth of a harmful gut bacterium called C. difficile, which can take hold when antibiotics clear out the competition. What makes this tricky is that it can develop up to two months or more after you’ve stopped taking amoxicillin, long after you’ve connected the dots to the antibiotic.
Other serious reactions listed by the National Institutes of Health include prolonged vomiting (starting one to four hours after a dose), extreme tiredness, and unusual paleness appearing within 24 hours of taking the medication. These symptoms are uncommon but warrant immediate medical attention. The same goes for any signs of a severe allergic reaction: difficulty breathing, swelling of the face or throat, or a rapidly spreading rash with blistering.
How Long Side Effects Last
Mild digestive side effects like nausea and loose stools typically start within the first couple of days and resolve within a few days of finishing your prescription. Your gut microbiome takes longer to bounce back. Research in animals shows significant disruption to bacterial communities and their metabolic output during treatment, though most of the changes are reversible over time.
Non-allergic rashes, especially those linked to viral infections, usually fade within a week or two without treatment. Yeast infections may appear during the antibiotic course or shortly after and generally respond quickly to antifungal treatment. The main outlier is C. difficile-associated diarrhea, which can emerge weeks after your last dose and may require its own course of treatment to resolve.
Reducing Your Risk
You can’t eliminate the possibility of side effects, but a few practical steps lower the odds. Eating something before or alongside your dose helps buffer your stomach. Finishing the full prescribed course, even if you feel better, prevents the need for repeat treatments that compound your risk. Each additional course of antibiotics means another round of disruption to your gut bacteria, and the meta-analysis data showed that harm from antibiotics is cumulative: for amoxicillin-clavulanic acid, roughly 1 in every 10 courses causes diarrhea.
If you’ve been told you’re allergic to penicillin based on a rash years ago, it may be worth getting formally evaluated. The vast majority of people with that label on their chart can safely take amoxicillin, and having access to it matters because it remains one of the most effective and affordable antibiotics available for common infections like strep throat, ear infections, and sinus infections.