Medication side effects, adverse effects and allergies

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.

Definitions

Side effects: The undesired effect that occurs when the medication is administered, regardless of the dose. Most side effects are expected and are independent of the dose. Some may be beneficial. Side effects often resolve on their own.
Adverse effects or reactions: The unwanted and undesirable effects related to a drug. Some are dose-dependent, and some are unpredictable. Often requires treatment.
Allergies: The reaction of the immune system to the drug. It is not dose dependent.

Common side effects and adverse effects of drugs

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.