DRUG ACTIONS AND INTERACTIONS
When a drug enters the body, it produces its effect by interacting with a specific target, or
receptor. A drug may cross the cell membrane to reach its intracellular receptor or may
react with a receptor on the cell's surface. The dose of a drug is the amount of drug
administered, usually measured in milligrams or grams. Schedule is the exact timing and
frequency of drug administration.
Various actions and interactions of drugs in the body can occur after they have been
absorbed into the bloodstream.
Additive action. If the combination of two similar drugs is equal to the sum of the effects
of each, then the drugs are called additive. For example, if drug A gives 10% tumor kill as
a chemotherapeutic agent and drug B gives 20% tumor kill, using A and B together would
give 30% tumor kill.
If two drugs give less than an additive effect, they are called antagonistic. If they produce
greater than additive effects, they are synergistic (as described next).
Synergism. A combination of two drugs sometimes can cause an effect that is greater than
the sum of the individual effects of each drug given alone. For example, INH (isoniazid)
and rifampin, two antibiotic drugs, are given together in the treatment of tuberculosis
because of their synergistic action to cure the disease. Individually, the drugs are not as
Response. This is a desired and beneficial effect of a drug. Lowering blood pressure by
antihypertensive drugs is an example.
Tolerance. For some drugs, the effects of a given dose diminish as treatment continues,
and increasing amounts are needed to produce the same effect. Tolerance is a feature of
addiction to drugs such as morphine and meperidine hydrochloride (Demerol). Addiction
is the physical and psychological dependence on and craving for a drug and the presence
of clearly unpleasant effects when that drug or other agent such as a narcotic is stopped.
Controlled substances are drugs such as opioids or narcotics that produce dependence and
have potential for abuse or addiction. See pages 906-907 in the Practical Applications
section for information about these drugs.