In order to somewhat simplify the presentation, cases were considered with the use of only one drug. Pharmacology also studies the effects of taking two or more psychoactive substances. Several drugs may be taken at the same time, or some next drug may be taken before the previous one is removed from the body.
Taking two or more drugs at once complicates the nature of the end result, since several drugs can interact with each other while being administered into the body. An interaction occurs when the action of one drug affects the action of another. Such interaction can be considered qualitatively and quantitatively. In pharmacology, the quantitative aspect is much better studied, that is, the strength of action of substances. In this sense, there are two types of interaction: the reinforcing effect of drugs and reducing it.
Combinations that enhance the effect. The term “synergism” is used to denote the interaction between drugs, which enhances their effect. If two specific drugs are synergistic, then the effect of their joint use will be greater than the sum of the effects of their separate use. In practice, it is difficult to determine whether certain drugs are synergistic, that is, whether the final effect is the simple result of adding the effects of two substances, or whether the drugs somehow reinforce each other.

In quantitative studies, drug interactions are presented as changes in the line of action of the dose. The case of synergism between the two drugs A and B is presented in Figure 4-7. The solid line is a typical dose response curve for a drug, which was analyzed in Figure 4-2. Drug synergism shifts this curve to the left (dashed line in the figure). It can be seen that if there is a drug B, drug A has a stronger effect.
Combinations that reduce the effect. The weakening of the effect of one drug in the presence of another in the body is indicated by the term “antagonism”. It is easy to guess that in case of antagonism of two drugs, the line of action of the dose in Figure 4-7 will move to the right. For example, amphetamines that stimulate the nervous system, are antagonists of alcohol and reduce its depressive effects. However, they do not interfere with alcohol to impair coordination of movements.

Importance of drug interaction. Knowledge of the nature of the interaction of substances is very important in medicine, since many drugs can be used in combination with others for more effective treatment of diseases. Thus, this interaction plays into the hands of medicine. On the other hand, it can create doctors and problems. Difficulties usually arise in the case when the doctor, prescribing the patient the medicine, does not know what other medicines he takes. One substance may negate the therapeutic effects of another. Moreover, the combination of certain drugs can have unhealthy or even deadly effects.
The interaction of substances in the body creates problems with the combination of drugs taken for treatment and other drugs. The most common situations of this kind are the mixing of substances that suppress the nervous system (depressants). For example, the simultaneous intake of alcohol and barbiturates can be fatal, since these substances enhance the sleeping effect of each other. This combination is often used for suicide, but sometimes death occurs as a result of careless mixing of these substances.

Drug interactions significantly affect the causes and nature of their use. People can deliberately mix drugs with the same or similar effect to get a “super effect”. It happens, on the contrary, used drugs with opposite effects to weaken the action of one of them. In rehabilitation centers, you can often meet people who have used alcohol with its depressive effects to overcome excessive stimulation caused by cocaine. Often they drink a lot of black coffee after drinking. Caffeine is an antagonist of alcohol, and it partly relieves the depressive effect of the latter on the central nervous system. These examples show actions related to the quantitative effect of drugs: people try to change the power of their actions. By mixing drugs, one can also achieve qualitative interaction between them. So,if a person takes a depressant (say, some of the benzodiazepines) along with LSD, then as a result he will experience complete peace of mind combined with a change in perception that will cause LSD.