Amoxicillin: learn about side effects, dosage, special precautions, and more on MedlinePlus Amoxicillin is a penicillin antibiotic that commonly causes mild side effects such as diarrhea, stomach upset, headache, an abnormal taste, skin rash, and vaginal yeast infections. 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. Amoxicillin can cause various side effects. These include nausea, headache, and diarrhea. Although most amoxicillin side effects are not serious, some people have reported life threatening. 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. 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. Amoxicillin is a type of penicillin antibiotic that treats bacterial infections. Common brand names include Amoxil®, Moxilin®, Sumox and Trimox. Learn about the side effects of amoxicillin, from common to rare, for consumers and healthcare professionals. 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.