Combined Effects of Drugs
When two or more drugs are given together, there is a possibility that the effect may be additive or synergistic. When the total effect of two or more drugs given together is same as that of their individual administration, the effect is called as additive. Sometimes the effect is more than expected, i.e. more than additive; in such cases it is called as synergistic effect. The risk associated with benzodiazepines like diazepam increases significantly when taken with alcohol in comparison to opiates like morphine or stimulants like amphetamine being taken alone. This is expressed by the comment that benzodiazepines are synergistic with alcohol. The explanation for synergistic effect is provided by different mechanisms of actions exhibited by involved drugs. The opposite of synergy is termed as antagonism. Two drugs are antagonistic when their interaction causes a decrease in the effects of one or both of the drugs.
Both synergy and antagonism can occur during different phases of the interaction of the drug with an organism, with each effect having different name. When the synergy occurs at a cellular receptor level, it is termed as agonism, and the substances involved are termed as agonists. On the other hand, in case of antagonism, one of the substances may be agonist and another antagonist. Different responses of receptors to the action of a drug has resulted in a number of classifications such as partial agonist, competitive/non-competitive agonist, etc. These concepts have fundamental applications in the phamacodynamics of these interactions.