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Pharmacology of drugs

Drug receptors

For drugs to work, they have to somehow produce a change in the functioning of a cell, which then leads to a change in body function or behaviour. Before it can do this, a drug must physically interact with one or more parts of the cell.

The cell component directly involved in this initial action of a drug is called the drug receptor. One characteristic of the drug receptor is its high, but not absolute, degree of specificity for a drug molecule.

What may appear to be a slight or insignificant variation in the chemical structure of a drug may greatly alter the intensity of the cell's response to the drug. For example, amphetamine and methamphetamine are both powerful stimulants of the central nervous system. They differ only slightly in chemical structure, however, methamphetamine is much more potent. Both drugs probably affect the same receptor in the brain, however, methamphetamine exerts a much more powerful action on that receptor.

Drugs and their effects

Pharmacology refers to the actions of drugs on mechanisms in the body. The effect of a drug is proportional to the concentration of that drug at its site of action.

Drug abuse refers to the use, usually by self-administration, of any drug in a manner that deviates from the approved medical or social patterns within a given culture. This definition is largely a social one. For any given drug, there is a large variation in what is considered abuse, not only from culture to culture, but also from one generation to another. Alcohol is a good example of this, as is the use of cigarette tobacco.

Sometimes in a society, the use of a drug may not be considered abuse until use of the substance becomes widespread. It is only after substantial research and development of social awareness that the use of certain drugs is viewed as drug abuse.

Drug tolerance

Drug tolerance is basically the body's ability to adapt to the presence of a drug.

The magnitude of the body's response to a particular drug depends on two factors:

  • Concentration of the drug at its site of action
  • Sensitivity of the target site to the drug

The sensitivity of the target cells is governed by genetic factors and adaptive changes by the body. Adaptive changes occur in response to the repeated exposure to a particular drug. The result is usually a loss of sensitivity to the drug. This decreased response is called tolerance

Tolerance may be defined as a state of progressively decreased responsiveness to a drug as a result of which a larger dose of the drug is needed to achieve the effect originally obtained by a smaller dose.

Drug dependence

There are two types of drug dependence.

Physical dependence

Physical dependence is a condition in which the body has adjusted to the presence of a drug, resulting in clear symptoms of withdrawal when its use stops. In extreme cases, the effect of rapid withdrawal can be life threatening because the body has become so dependent on the drug as to interfere with normal body processes.

An individual physically dependent on a drug requires that drug in order to function normally. Physical dependence is associated with tolerance in most cases. The state of physical dependence is revealed by withdrawing the drug and noting the occurrence of withdrawal symptoms some time after the drug is withheld. The symptoms of withdrawal can be terminated by re-administration of the drug.

Symptoms of drug withdrawal tend to be the opposite of the effects of the drug. If the effect of the drug is sedation, the withdrawal effect will likely be hyperexcitability. If the effect was stimulation the withdrawal effect will likely be be emotional depression.

Longer acting drugs tend to produce less intense withdrawal symptoms because the body has more time to adapt to the decreasing presence of the drug.

Psychological dependence

This kind of dependence is characterized by emotional and mental preoccupation with the drug's effects and by a persistent craving for it.

The symptoms displayed are not physical symptoms. Craving seems to be the most common withdrawal symptom.

Psychological dependence is usually manifested by compulsive drug-taking, but the frequency and pattern of use can differ considerably from one individual to another.

Cross Dependence

This is the ability of one drug to suppress the manifestations of physical dependence produced by another and to maintain the physically dependent state. Cross dependence may be partial or complete. One amphetamine will show cross dependence with other amphetamines. Most sedatives show cross dependence with each other and with alcohol. Cross dependence usually occurs among compounds of a given family of drugs but may also occur among drugs of different families that have similar pharmacological effects.


Addiction is a difficult word to define since it can be used in various ways. The World Health Organization (WHO) has provided the following definition: "A behavioral pattern of drug use, characterized by overwhelming involvement with the use of a drug (compulsive use), the securing of the supply, and a high tendency to relapse after withdrawal. Addiction is viewed as an extreme on a continuum of drug use patterns. It refers, in a quantitative rather an a qualitative sense, to the degree to which drug use pervades the total life activity of the user, and to the range of circumstances in which drug use controls his/her behaviour."

Addiction refers to dependent patterns of drug self-administration without making a distinction between physical or psychological dependence. Moral weakness is often implied by the term addiction.

The WHO has suggested that the term "addiction" be replaced with the term "drug dependence." It is not possible to identify with precision the point where compulsive use should be considered addiction.

The term addiction cannot be used interchangeably with physical dependence since one can be physically dependent on drugs without being addicted and, in some cases, addicted without being physically dependent.

Self-administration of drugs depends upon a number of factors. These include:

  • the properties of the drug itself
  • the route of administration
  • the size of the individual dose
  • the amount of work required to obtain a dose
  • the presence of other drugs
  • previous experience with other drugs

With continuous access, animals show patterns of self-administration that are strikingly similar to those exhibited by human users of the same drug. Scientific studies have shown that pre-existing mental and behavioral disorders are not a prerequisite fpr drug use and that drugs themselves are powerful reinforcers, even in the absence of physical dependence.

Learning, conditioning, and relapse

Drug use can be viewed as behaviour that is maintained by its consequences. Consequences that strengthen a behaviour pattern are reinforcers. Drugs may reinforce a behaviour by inducing pleasurable effects, or terminating unpleasant effects. They may also supply social reinforcement.


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