Medicines (medicines). By origin, drugs are divided into the following groups: 1) products of chemical synthesis; 2) waste products of microorganisms (antibiotics, some vitamins); 3) substances of plant origin (from medicinal plants);
4) substances of animal origin (hormonal, enzyme preparations);
5) substances derived from inanimate nature (minerals, products of coal, oil, etc.). According to the mechanism of the therapeutic or prophylactic action, drugs can be divided into two large groups: some have this effect by changing the functions of certain physiological systems of the body, others - by destroying microbes or parasites - the causative agents of diseases. In the first case they talk about pathogenetic, in the second - about etiotropic therapy.
The action of drugs is carried out as a result of their interaction with certain biochemical (reactive) structures of the body, which leads to a change in the course of biochemical processes, and at the same time the function of the corresponding tissues and organs. Depending on the characteristics of the chemical structure of the drug react with certain reactive structures, and since the nature of the latter in different tissues and cells is not the same, drugs can have a selective or preferential effect on certain tissues and cells. For the same reason, drugs acting on pathogens selectively cause death or suppress the reproduction of some species of microbes or parasites and have little effect on other species.Go
As a rule, medicinal substances form weak bonds (electrostatic, van der Waalsa, hydrogen) with the biochemical structures of the body. Therefore, the effect of drugs is reversible. Only in some cases, the pharmacological substance (for example, alkylating anticancer agents, some organophosphorus compounds) forms strong covalent bonds with the reactive structure; however, the drug acts until biological resynthesis of the corresponding reactive structures occurs.
Types of action of drugs . The effect of the drug, which is manifested before it is absorbed into the blood, is called local (pre-resorptive), after absorption - resorptive. The direct effect of a drug is an effect, which is a direct result of the effect of a drug on reactive structures. Due to the interrelationships between individual organs, changes in the function of any of them, as a rule, lead to more or less pronounced changes in other organs and physiological systems of the body. Therefore, the direct action of drugs is accompanied by indirect effects, or secondary reactions to the pharmacological effects of the organs and systems that are not directly affected by the drugs. For the occurrence of indirect effects of the drug is typical reflex mechanism. In such cases, indirect effects are called the reflex action of the drug.
Along with the desired effect of a drug, undesirable effects may also occur — a side effect that may vary depending on the chemical nature of the drug used and the characteristics of its interaction with the body. Among the serious side effects of drugs is their teratogenic effect on the fetus, which can be observed when prescribing drugs to pregnant women and is a violation of the processes of laying and the development of tissues and organs of the fetus, leading to congenital deformities (see Embryopathy). The teratogenic properties of drugs have become known recently and are still little studied; therefore, during pregnancy, especially in its initial period, it is necessary to carefully apply drugs, especially if there are no serious indications for their use.Go
Ways of introducing drugs into the body . The resorptive effect of drugs can be achieved by enteral or parenteral administration. The main methods of enteral - ingestion and administration through the rectum. When ingested all absorbed medication enters primarily in the liver, where it can undergo chemical transformations. When absorbed from the rectum, the drug partially enters the liver through the portal vein system, and partially through the inferior vena cava directly to the heart. Thus, with this method of administration, part of the absorbed drug enters the general circulatory system without undergoing preliminary chemical transformations in the liver. Parenteral administration methods (subcutaneous, intramuscular, inhalation, intravenous, etc.) provide faster absorption and a higher concentration of the drug in the blood.
Methods of administering drugs to achieve local action are very diverse: drugs are applied to the skin, injected into the conjunctival sac, into the nasal cavity, into the external auditory canal, into the vagina, into the rectum, taken orally (for example, astringent, laxatives, etc.) ), are injected subcutaneously (for example, local anesthetic substances), into the joint cavity, into the serous cavity, into the subarachnoid space, etc. It should be borne in mind that when using drugs to achieve local action, sometimes also resorptive effects are obtained, vpl It is up to phenomena of poisoning. The intensity of the pharmacological reaction with both local and resorptive action depends on the concentration of the drug in the immediate vicinity of the reactive structures with which it interacts. With resorptive action, this concentration primarily depends on the dose administered.
Isolation of drugs . Excretion of drugs from the body occurs mainly through the kidneys. The rate of excretion of various drugs by the kidneys is not the same. As a rule, lipid-soluble drugs are retained longer in the body due to greater reabsorption in the renal tubules. On the contrary, drugs that are poorly soluble in lipids and well soluble in water, especially highly dissociated substances, leave the body relatively quickly, since they are less subject to reabsorption in the renal tubules. Medicines that undergo active excretion of the renal tubule epithelium (for example, penicillin) are most rapidly excreted from the body. Some drugs are also secreted by the glands of the digestive tract and can be reabsorbed from the intestines (for example, morphine, mercury compounds, phenolphthalein, etc.). Volatile substances are released through the respiratory tract along with exhaled air. It is practically important that medicines are also excreted by the mammary glands; This should be taken into account when prescribing medicines for lactating women.
The transformation of drugs in the body . Chemical transformations of drugs occur mainly in the liver. As a rule, lipid-soluble substances undergo transformations. At the same time, products having a more polar structure are formed from them, as a result of which they are less likely than the original drug, undergo reabsorption in the renal tubules and leave the body more quickly. Thus, the ability of an organism to subject chemical transformations to medicinal substances is a particular manifestation of the adaptation developed in the course of evolution to accelerate the release of substances that are alien to the organism and which enter it from the external environment. Chemical transformations of drugs are catalyzed by enzymes of low specificity, which are mainly contained in liver cell microsomes. In most cases, chemical transformations of drugs lead to their inactivation, however, often as a result of these transformations, drugs acquire pharmacological activity (for example, chemotherapeutic drugs from a number of arsenobenzenes, bighumal, phenacetin, etc.).
Sensitivity to drugs depends on a number of factors, of which almost the most important age. Children are generally more sensitive to medications than adults, and to achieve pharmacological reactions of the same intensity, children have lower doses of medications (per 1 kg of body weight) than adults. Higher sensitivity of children to drugs depends on the characteristics of the physiological regulation in the children's body: in children, compensatory mechanisms are less perfect, which level the changes in functions that develop under the influence of drugs. In children, the activity of those liver microsome enzymes that are involved in the chemical transformations of drugs is lower. With regard to drugs that are not subjected to chemical transformations in the body, the sensitivity of children is close to the sensitivity of adults. To some pharmacological agents, in particular to substances that cause hypoxia, children are less sensitive than adults.
Sensitivity to many drugs (sleeping pills, analgesics of the morphine group, nitroglycerin, laxatives, etc.) decreases when they are reintroduced - the so-called addiction. The addiction to drugs acting on higher nervous activity is often accompanied by painful addiction. Medications that persist for a long time in the body (for example, long-acting barbiturates, bromides, cardiac glycosides from digitalis, etc.), when they are used again, accumulate in gradually increasing concentrations, which can lead to toxic complications. This phenomenon is called cumulation.
Some individuals experience idiosyncrasy (see) to certain drugs, such as iodine, aspirin, some antimalarial drugs, etc. At the same time, the usual doses of drugs cause inflammation of the mucous membranes, skin rashes, hemodynamic disturbances, hemolysis, etc. ( see Drug Disease).
In the case of the combined action of drugs, their synergism is most often used, i.e., the action is in one direction, thus enhancing the therapeutic effect. The variant of synergism is especially valuable - potentiation (synergism in which the effect of the combined action of drugs is greater than the sum of the effects of each of the components taken separately). To eliminate the side effects of drugs, sometimes antagonism of drugs is used, in which one drug prevents the action of another. Thus, in the treatment of epilepsy with phenobarbital to eliminate the hypnotic effect of the drug, it is sometimes prescribed in combination with caffeine or phenamine, which are antagonists of phenobarbital as a hypnotic, but do not interfere with its antiepileptic effect. Especially often the antagonism of drugs used in the treatment of acute poisoning.