The most common side effects of amoxicillin are skin rash, nausea, vomiting, and diarrhea. It may also cause allergic reactions or serious side effects, but those are less common. Learn about the side effects of amoxicillin, from common to rare, for consumers and healthcare professionals. Common side effects include diarrhea, rash, nausea, or vomiting. Serious side effects are rare but may include severe diarrhea, allergic reactions, or skin reactions. Amoxicillin (Amoxil) is typically well-tolerated, but nausea, vomiting, and diarrhea are some of its most common side effects. In rare cases, more serious amoxicillin side effects are also possible. Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention. However, amoxicillin can cause side effects, allergic reactions, and serious problems if the dose is wrong, the drug is not needed, or human products are used without a veterinarian’s guidance. Side effects of amoxicillin are typically mild and include gastrointestinal issues like upset stomach and diarrhea, as well as headache, rash, abnormal taste, and vaginal yeast infections.