Antibiotics are among the most commonly prescribed medications in the US. An antibiotic side (adverse) effect is an unwanted reaction that occurs in addition to the desirable therapeutic action of the antibiotic. When used appropriately, antibiotics are relatively safe with very few side effects. However, like any drug, antibiotic side effects can occur and may interfere with the patient’s ability to tolerate and finish the course of medication.

Antibiotic reactions can range from mild allergic reactions to severe and debilitating adverse events. Antibiotic side effects are extremely variable from patient to patient and from antibiotic to antibiotic.

If a patient is experiencing a bothersome or serious antibiotic side effect, they should contact their health care provider. The outcomes may include staying on the same antibiotic and managing the side effect, adjusting the dose, or switching to a different antibiotic. Usually, antibiotic treatment should not be stopped without a health care provider’s approval. Stopping the antibiotic may allow the infection to worsen and may lead to antibiotic resistance. Even if the infection appears to have cleared up before all of the medication is gone, the full course of antibiotic treatment should always be completed unless told otherwise by a health care provider.

An antibiotic allergy or hypersensitivity reaction can happen with any drug, and allergies are one of the most common antibiotic side effects leading to emergency room admission.1 Health care providers should always be informed of any previous allergic reaction to any medication, including antibiotics. Mild allergic reactions may only result in a skin rash. More severe allergic reactions, called anaphylaxis, can lead to shortness of breath, wheezing, hives, and swelling of the face, lips or tongue. Anaphylaxis is a medical emergency that requires immediate medical attention.

Antibiotics are used to kill bacterial infections; they are not effective against viral infections, such as a cold or the flu, or against fungal infections, like ringworm or vaginal yeast infections. The most common antibiotic classes and drug members are listed in Table 1, along with the most commonly reported antibiotic side effects. This is not a complete list of all available antibiotics or side effects that may occur.