Drugs can be eliminated directly from the body or first decompose into substances whose metabolites are less likely to be re-absorbed. These secondary substances are also excreted from the body. Liver enzymes play a major role in drug metabolism. It is important that these enzymes are also found in other organs, such as the kidneys and the digestive tract. Therefore, a drug ingested by mouth undergoes partial decomposition by enzymes. Thus, less substance reaches the required place than was accepted. Some drug metabolites are pharmacologically active. They cause, for example, side effects of various drugs. There are two examples of metabolites with psychoactive properties: these are diazepam (Valium) and chlordiazepoxide (Librium) metabolites. One of the metabolites of diazepam is removed from the body more slowly,than diazepam itself. Their action is terminated by further metabolism or elimination from the body along with urine.

The main organ responsible for the removal from the body of both drugs and their metabolites is the kidneys. But removal can go in other ways. For example, drugs ingested by mouth are sometimes excreted along with feces. Drug metabolites can be removed along with bile.
Drugs are released in human milk, and although the quantity of drug withdrawn in this way is small, the infant is at serious risk. Drugs can also be removed through the lungs: this is why the breath of a person who has taken alcohol smells like alcohol. Finally, drugs are removed with sweat. Drug testing Discussion of the process of removing drugs from the body brings us to the question of how to test a person for drugs. There are many methods for determining whether a person has taken drugs using urine or blood tests. Urinalysis is more sensitive because there can be found metabolites of the drug. If the drug is found in the blood, it indicates a very recent use of it.

The success of the test depends on many factors, including the magnitude of the last dose taken, the specific test method and the exact procedure followed. The ability to detect a drug depends mainly on the rate of its elimination from the body and the rate of elimination of its metabolites. Table 4-4 shows the time of excretion of drugs (and their metabolites) from the body and the time after which it is possible to detect them in the body after use. Less time in the second column corresponds to the removal of the drug itself from the body, more time – its metabolites.
For alcohol, only one time is indicated, since its metabolites are widely distributed in the body, and the alcohol test for its metabolites will be unreliable.

The third column indicates the maximum time during which the test can produce results. This time is greatly increased if the content of drug metabolites in the body can be measured. This time is usually much shorter than the period of excretion of drugs and their metabolites from the body. This is because testing has limited sensitivity and cannot detect the content of metabolites after a certain limit. This limit depends on the specific metabolite and testing methodology. This means that a positive review of pharmacokinetics has led you to the result; however, it does not inform us of the exact time of drug use. The exception is, as noted, the detection of a drug in the blood, which indicates a very recent use.Traces of a large dose of marijuana can be detected within eight days of use. In the body of people who constantly use marijuana, you can find traces of it more than a month after taking the last dose.

Farcodynamics 

Let us turn to understanding the mechanism of action of drugs. This is a subject of pharmacodynamics. In the remainder, terms will be introduced in which pharmacologists describe the effects of drugs, as well as graphical expressions of such effects.