When drugs are ingested or administered into the body, they are absorbed into the bloodstream or into the body tissues. The drugs then combine with, or alter, the molecules in the body cells, changing the way the cells work. How the drugs produce these changes within the body is known as drug action.
The following terms describe the action and interaction of drugs in the body after they have been absorbed into the bloodstream:
Additive Action. The combination of two similar drugs is equal to the sum of the effects of each. For example, if drug A gives 10 per cent tumor kill as a chemotherapeutic agent and drug B gives 20 per cent tumor kill, using A and B together would give 30 per cent tumor kill.
Cumulative Action. After administration of certain drugs, concentrations of the drug or its toxic effect on tissue may increase with each dose. The drug dose may have to be reduced to prevent accumulation to toxic concentrations.
Idiosyncrasy. This is any unexpected effect that may appear in the patient following administration of a drug. Idiosyncratic reactions are produced in very few patients but may be life- threatening in those few instances. For example, in some individuals penicillin is known to cause an idiosyncratic reaction such as anaphylaxis (acute type of hypersensitivity, including asthma and shock).
Synergism (Potentiation). A combination of two drugs can sometimes cause an effect that is greater than the sum of the individual effects of each drug given alone. For example, penicillin and streptomycin, two antibiotic drugs, are given together in treatment for bacterial endocarditis because of their synergistic effect. Tolerance. The effects of a given dose diminish as treatment goes on, and larger doses must be given to maintain the desired effect. Tolerance is a feature of addiction to drugs such as morphine and merperdine hydrochloride (Demerol).