A medicine is any drug or other agent used to treat or prevent disease or to treat injury. The term medicine is generally used as a synonym for “drug.” In devising medicines for treating common conditions, an especially desirable factor is that the medicine should be capable of being taken by mouth that is, that it should be able to pass into the body unchanged in spite of being exposed to stomach acidity and the enzymes of the digestive system.
In many cases, this is possible but there are some important exceptions, such as insulin that has to be given by injection.
This method may also be necessary if vomiting or a disease of the stomach or intestine prevent normal absorption. In most cases, the level of the drug in the blood or tissues determines its effectiveness.

Factors that can make a difference in the effectiveness of a drug include:
- the route of administration
- the rate of distribution in the body
- the degree of binding the proteins of
the blood plasma
- the rate of breakdown
- the rate of excretion
- interactions with other drugs

In addition, there is an individual variation in drug responsiveness that is also apparent with undesired side-effects. These arise because drugs acting on one system commonly act on others.

Side effects

may be of various kinds, either non-specific, allergic or specific: - non-specific side effects include nausea, vomiting, diarrhea, malaise or skin reactions;
- allergic side effects include urticaria (nettle rash), skin reactions and anaphylaxis (hypersensitivity leading to shock);
- specific side effects are related to the action of the medicine on the organ to be treated such as abnormal heart rhythm as a side effect of digitalis. Mild side effects may be suppressed or simply accepted, but more serious ones must be watched for and the medicine stopped at the first sign of any adverse effect. Drugs may cross the placenta to reach the fetus during pregnancy, interfering with its development and perhaps causing deformity, as happened with thalidomide