Drugs are administered to obtain a certain pharmacotherapeutic effect: for example, to relieve pain. Hypotensive drugs are used to decrease arterial blood pressure, etc. This is a manifestation of a drug's principal effect due to which they are used in practical medicine. However, along with desirable effects all drugs have adverse effects, including non-allergic side effects, allergic reactions, toxic and other effects.
Non-allergic side effects include only those effects that occur in the administration of therapeutic doses and that are in the range of their pharmacological action (see Fig. II.16). Thus, phenobarbital, administered as an antiepileptic drug, may cause drowsiness. Pain-relieving morphine in therapeutic doses causes euphoria and stimulates the tone of gastrointestinal sphincters.
Side effects can be primary and secondary. Primary side effects occur as a direct effect of the drug on a certain substrate (for example, nausea and vomiting occurring due to the irritant action of the drug on the gastric mucosa). Secondary side effects refer to adverse reactions developing indirectly (for example, hypovitaminosis due to suppression of the intestinal flora with antibiotics).
Adverse effects of drugs vary in character, intensity and duration. Side effects may target the nervous system, blood and hematopoiesis, organs of blood circulation, respiration, digestion, as well as kidneys, endocrine glands, etc. Some side effects are tolerated relatively easily (moderate nausea, headache, so on); others can be severe and even life threatening (liver damage, leucopenia, aplastic anemia).