The quantity of a medicine required for its optimal effect. Many factors influence this, including age, weight, sex, idiosyncrasy, genetic disorders, habitual use, disease, fasting, combination with other drugs, the form in which the drug is given, and the route by which it is given.
Normally, a young child requires a smaller dose than an adult. There are, however, factors other than age to be taken into consideration. Thus, children are more susceptible than adults to some drugs such as MORPHINE, whilst they are less sensitive to others such as ATROPINE. The most precise way to calculate a child's dose is by reference to texts supplying a recommended dose according to weight or body surface area. Older people, too, often show an increased susceptibility to drugs. This is probably due to a variety of factors, such as decreased weight; diminished activity of the tissues and therefore diminished rate at which a drug is utilised; and diminished activity of the KIDNEYS resulting in decreased rate of excretion of the drug.
Women require slightly smaller doses than men, probably because they tend to be lighter in weight. The effect of weight is partly because much of the extra weight of a heavy individual is made up of fatty tissue which is not as active as other tissue of the body. In practice, the question of weight seldom makes much difference unless the individual is grossly over- or underweight.
Occasionally drugs administered in an ordinary dose produce an unexpected effect. Thus, in some people certain drugs – for example, psychoactive preparations such as sedatives – produce excessive symptoms in normal or even small doses. In some cases this may be due to hypersensitivity or to an allergic reaction to the drug, which is a possibility that must always be borne in mind (e.g. with PENICILLIN). In many it is becoming clear that their genetic makeup affects the way they metabolise certain drugs. An individual who is known to be prone to an idiosyncratic reaction to certain medication is strongly recommended to carry a card to this effect, and always to inform medical and dental practitioners and/or a pharmacist before accepting a new prescription or buying an over-the-counter preparation.
of a drug causes the greatest increase in the dose necessary to produce the requisite effect. The classical example of this is with HEROIN and its derivatives.
may modify the dose of medicines. This can occur in several ways. Thus, in serious illnesses the patient may be more susceptible to drugs, such as narcotics, that depress tissue activity, so smaller doses must be given. Absorption of the drug may be slowed up by disease of the gut, or its effect may be enhanced if there is disease of the kidneys or liver, interfering with the excretion of the drug.
aids the rapidity of absorption of drugs, and also makes the body more susceptible to their action. Partly for this reason, as well as to avoid irritation of the stomach, it is usual to prescribe drugs to be taken after meals, and diluted with water.
is to be avoided if possible as it is often difficult to assess what their combined effect may be. In some cases they may have a mutually antagonistic effect, which means that the patient will not obtain full benefit.
Drugs are produced in many forms, though tablets and capsules are the most common. Medicines are given by mouth whenever possible, unless there is some degree of urgency, or because the drug is either destroyed in, or is not absorbed from, the gut. In these circumstances it may be given by injection, intravenously (into a vein), intramuscularly (into muscle) or subcutaneously (under the skin). In some cases, as in cases of ASTHMA or BRONCHITIS, the drug may be given in the form of an INHALANT, in order to get the maximum concentration in the lungs. If a local effect is wanted, as in cases of diseases of the skin, the drug is applied topically (to the skin). In some countries there is a tendency to give medicines in the form of a suppository which is inserted in the rectum.
Recent years have seen developments whereby the assimilation of drugs into the body can be more carefully controlled. These include, for example, transdermals, in which drugs are built into a plaster that is stuck on the skin; the drug is then absorbed into the body at a controlled rate. This method is now being used for the administration of GLYCERYL TRINITRATE in the treatment of ANGINA PECTORIS, and of hyoscine hydrobromide in the treatment of MOTION SICKNESS. Another method involves implantable devices. These are tiny polymers infused with a drug and implanted just under the skin by injection. They can be tailored so as to deliver drugs at virtually any rate – from minutes to years. In yet another new development, a core of drug may be enclosed in a semi-permeable membrane and is released in the stomach at a given rate. (See also LIPOSOMES.)