An adverse effect may result from an unsuitable or incorrect dosage or procedure (which could be due to medical error). Some adverse effects occur only when starting, increasing or discontinuing a treatment. Using a drug or other medical intervention which is contraindicated may increase the risk of adverse effects. Patients sometimes quit a therapy because of its adverse effects. The severity of adverse effects ranges from nausea to death. Common adverse effects include alteration in body weight, change in enzyme levels, loss of function, or pathological change detected at the microscopic, macroscopic or physiological level.

 Adverse effects may cause a reversible or irreversible change, including an increase or decrease in the susceptibility of the individual to other chemicals, foods, or procedures (e.g. drug interaction).