The most common side effects of amoxicillin are digestive: diarrhea, nausea, and stomach pain, which affect roughly 1% to 10% of people taking the drug. Most side effects are mild and resolve on their own after finishing the course, but amoxicillin can also cause allergic reactions and, rarely, more serious problems that warrant immediate attention.
Digestive Side Effects
Diarrhea is the side effect people experience most often. Amoxicillin disrupts the normal balance of bacteria in your gut, which can loosen stools or cause outright diarrhea from the first day of treatment through several weeks after stopping. Nausea and abdominal pain fall in the same 1% to 10% range, while vomiting is less common, occurring in fewer than 1 in 100 people.
These symptoms are usually manageable. Taking amoxicillin with food can reduce nausea for many people. A meta-analysis of 36 trials with over 9,300 participants found that taking probiotics alongside antibiotics reduced the incidence of antibiotic-associated diarrhea by 38%, with no increase in adverse events. Starting probiotics early in the antibiotic course appears to offer the strongest protection, though spacing them a couple of hours apart from your amoxicillin dose is a common recommendation.
Skin Reactions and Rashes
Rashes are one of the trickier side effects of amoxicillin because they can mean very different things. A flat, non-itchy rash that appears around days 7 to 10 of treatment is common, especially in children, and is usually not a true allergy. In a study of 668 children seen in emergency or urgent care settings, hives appeared in about 44% of cases and a flat, measles-like rash in 36%, with most reactions showing up in that 7 to 10 day window.
True allergic reactions, however, can be serious. Hives that appear with itching, swelling of the face or throat, or difficulty breathing signal a potentially dangerous response. Fatal anaphylactic reactions have been reported with penicillin-type antibiotics, and people with a history of penicillin allergy or sensitivity to multiple allergens are at higher risk. If you develop a rash that spreads quickly, blisters, or is accompanied by fever and peeling skin, that could indicate Stevens-Johnson syndrome or another severe cutaneous reaction. These are rare but represent medical emergencies. Early warning signs include fever, sore mouth and throat, fatigue, and burning eyes one to three days before a rash appears.
Effects in Children
Children, especially infants, are prone to a specific side effect that adults rarely deal with: diaper rash. A study of 57 infants receiving amoxicillin for ear infections found that 10 days of treatment doubled the amount of yeast recovered from the skin and rectal area. That overgrowth of yeast directly increased the risk of developing diaper dermatitis. Frequent diaper changes and barrier creams can help, and antifungal creams may be needed if the rash has a bright red appearance with satellite spots, which suggests yeast involvement.
The FDA also added a warning in 2024 about drug-induced enterocolitis syndrome (DIES), a reaction seen mostly in children 18 and younger. DIES causes severe, prolonged vomiting one to four hours after taking a dose, without any skin rash or breathing problems. It can also involve paleness, lethargy, and diarrhea within 24 hours. This is a non-allergic hypersensitivity reaction, meaning it won’t show up on standard allergy tests.
Gut Infections From Disrupted Bacteria
Because amoxicillin kills beneficial gut bacteria alongside the harmful ones, it can create an opening for Clostridioides difficile, a bacterium that causes severe, watery diarrhea and colon inflammation. Amoxicillin is classified as a medium-risk antibiotic for C. diff, with about 2.5 cases per 10,000 prescriptions. That’s a low absolute number, but the risk is roughly double that of low-risk antibiotics like doxycycline.
C. diff diarrhea is different from ordinary antibiotic-related diarrhea. It tends to be more frequent (three or more watery stools a day), may contain mucus or blood, and is often accompanied by fever and cramping. It can develop while you’re taking amoxicillin or up to several weeks after finishing it. If diarrhea gets worse rather than better as your course continues, or starts after you’ve stopped the antibiotic, that’s worth bringing to your doctor’s attention promptly.
Liver Effects
Amoxicillin on its own rarely causes liver problems, but the combination of amoxicillin with clavulanate (sold as Augmentin) carries a more notable risk, estimated at 1 to 17 cases per 100,000 prescriptions. Liver injury from this combination can show up as jaundice, dark urine, or upper abdominal pain. The timing is unusual: it typically appears within four weeks of starting the antibiotic but often doesn’t become apparent until after you’ve already stopped taking it, sometimes up to 16 weeks later. This delayed onset can make it hard to connect the symptoms to the medication.
Allergic Reactions and Anaphylaxis
Amoxicillin belongs to the penicillin family, and penicillin allergy is one of the most commonly reported drug allergies. Mild allergic reactions like hives and itching are far more frequent than severe ones, but anaphylaxis, a whole-body allergic response causing throat swelling, a drop in blood pressure, and difficulty breathing, can occur and can be fatal. This risk is highest in people who have had previous reactions to penicillin-type drugs or who are sensitive to multiple allergens.
People of Asian and South Asian ancestry may carry a genetic variation (HLA-B*1502) that increases the risk of severe skin reactions to certain medications. The FDA recommends genetic screening for this variant before starting some related drugs, though this is more commonly applied to other medication classes than to amoxicillin alone.
Reducing Your Risk of Side Effects
Most people tolerate amoxicillin without significant problems. A few practical steps can lower your chances of the common ones:
- Take it with food to reduce nausea and stomach irritation.
- Use probiotics during and after your course. The evidence supports starting them as early as possible alongside the antibiotic.
- Finish the full course as prescribed. Stopping early doesn’t reduce side effects but does increase the risk of drug-resistant bacteria and incomplete treatment.
- Watch the timing of symptoms. Side effects that appear in the first few days and stay mild are generally expected. Symptoms that worsen over time, start after you finish the course, or involve blistering skin, facial swelling, or severe diarrhea need prompt medical evaluation.