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FACTORS THAT AFFECT DRUG ACTION




Many factors must be considered relative to the effect of drug action on the body. First, different drugs exert different effects on various body cells, and unless a substance is able to exert an influence on some body cell or process, no reaction to the substance will take place. Second, before a drug enters the body, three important factors will affect the response of the person to the drug, namely its dose the time it takes it to be effective, and the route of administration. Other factors, such as the individual's psychological environment, metabolism rate, excretion route, age, weight, sex, and hereditary influences, can also affect the action of any drug on the individual. The effect of a drug on the body depends upon (1) its interaction with a body cell and (2) the function of that cell in the body. In most cases, the drug must bond with the cell membrane in order for any action to take place. The point on the cell where the interaction takes place is called the receptor site. The drug-receptor site interaction has often been described as a "lock and key" relationship. The specificity of the drug action among chemically related drugs usually depends upon the degree of fit between the drug and its receptor molecule. The exact mechanism of how this works in the case of many drugs still is not known. Some drugs, however, act on the cell from the point of the receptor site in the following ways:

1. Increase activity of cell. 2. Decrease activity of cell. 3. Block activity of cell. 4. Replace missing component needed by cell. 5. Aid in transferring substance through cell membrane.

In some cases, after a drug enters the body, it must be chemically altered before it is able to exert any effect on the body cells. This can occur at the cellular receptor site or in the stomach or intestines.

Dose

The response differs for all drugs in the intensity and types of effects according to the dose (amount) of the drug given. Most drugs do not show an effect until a certain minimal level of dosage is reached. The smallest dosage of the drug to show an effect is the threshold of the drug. As the result of heredity, different people respond in a different manner to the same dosage of a drug. The dose of a drug required to produce an effective response in 50 percent of the people tested is called the median lethal dose. Obviously, this latter dosage is only given to experimental animals when testing drug safety. In most cases, the more of a drug one takes, the stronger effect of the drug. However, the increased dosage is often detrimental to the individual, as a lethal dosage may be reached. For example, if one wishes to get slightly high, to feel relaxed, and still remain in control, one might drink a glass of wine during one hour before dinner with a friend. This could be called the effective dose of the alcohol if this were the purpose of using this drug. To increase the dosage to three or six glasses of wine in an hour would probably result in such effects as decreased reaction time, slurring of speech, sleepiness, uncoordinated movements, difficulty in speaking, and a bad hangover the next day. A still greater number of glasses of wine in an hour could result in a coma or even in death. In the case of alcohol, an increase in the dose intensifies the reaction of the drug but does not increase the desired effect sought by the drug user. This would also be true of other drugs. For example, an increase in the dosage of penicillin for a "strep" sore throat will not cure the problem any faster.

Time

The time that it takes for a drug to exert its desired effect once it has entered the body is also important. Some drugs act immediately upon entering the circulatory system, and other drugs take hours and sometimes even days to exert their desired effects. Drugs that act immediately are termed short-term and often require only single doses, whereas other drugs must be given in repeated dosages to be effective and are often called long-term effect drugs. As an example, adrenalin (epinephrine) will act immediately upon inhalation or injection, whereas an antibiotic such as penicillin may take hours or even a day or so for it to be effective, no matter if it is given by injection or by mouth.


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