There can be various types of unintended effects after taking medication. These include side effects, adverse effects or reactions, and drug allergies. The terms side effect and adverse effect are often used interchangeably.
| Drug | Side effect or adverse effect |
| Insulin | Hypoglycemia |
| Anticoagulants (e.g. warfarin) | Bleeding |
| Antibiotics | Diarrhea |
| Opioid analgesics | Respiratory depression, addiction |
| Digoxin | Upset stomach, vomiting, vision changes |
| Phenytoin | Confusion, lack of coordination |
| Carbamazepine | Dizziness, drowsiness |
| Metformin | Lactic acidosis |
| Antidepressants | Weight gain |
| Chemotherapy | Hair loss, anemia |
Drug allergies commonly present with skin rashes, itching, hives, and sometimes wheezing, swelling, and fever. They may also present with joint swelling, a high eosinophil count, anemia, and blood in urine. Some patients may have serious allergies to drugs, which may cause anaphylaxis, which presents with trouble breathing, severe hypotension, weak pulses, seizures, and airway swelling.
Most drug reactions are mild to moderate. Severe reactions may rarely occur and include liver failure, renal failure, cardiac arrhythmias, immunosuppression, severe skin rashes, birth defects, etc.
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