Grapefruit, gingko and a peptic ulcer are all drug interactions

Most of the time medicines make life better: they reduce aches and pains,
fight infections, and control problems such as high blood pressure or
diabetes. They can also produce unwanted reactions, and those come in three
distinct kinds.

1. Drug interactions

An interaction is a change in the way a drug acts in the body because of
something else — and the something else is not always another drug:

An interaction can make a drug more or less effective, cause side effects, or
change how one or both drugs work. The second and third rows are the ones
people do not think to mention: a supplement bought over the counter and a
piece of fruit are both capable of changing what a prescription does.

2. Side effects

Unwanted, usually unpleasant effects of a medicine. Most are mild — a
stomachache, dry mouth, drowsiness — and go away when you stop taking it.
Others are more serious.

A drug can also interact with a disease you have and cause a side effect
through it: **with a heart condition, certain decongestants can cause a rapid
heartbeat.**

3. Drug allergies

A different kind of reaction, ranging from mild to life-threatening.
Skin reactions such as hives and rashes are the most common.
Anaphylaxis, a serious allergic reaction, is less common.

Staying safe

When you start a new prescription or over-the-counter medicine, make sure you
understand how to take it correctly, and **know which other medicines,
foods and supplements to avoid**.

That second half is the one this page exists for. Always ask your provider or
pharmacist if you have questions.

Source: MedlinePlus, U.S. National Library of Medicine.