Such drugs are called 'antagonists'.

A sympathetic neurotransmitter called noradrenaline released at the site of heart causes increase in the force and speed of the heart beat. This is not desirable in conditions like hypertension (high BP), angina (heart pain, etc.) Hence in such conditions a drug like propranolol, that is, a beta-blocker, is used. This is an antagonist to noradrenaline that is, it occupies the same beta-receptors as noradrenaline by displacing the latter and thus blocks noradrenaline's action resulting in a decrease in the force and speed of the heart beat.

II.     Dosage and Usage Information

Usual Dose or Dosage Range

Dose of the drug implies the amount of drug to be taken to produce the beneficial effect but not cause excessive harmful effects. If the dose is too low, it may not have any effect at all; if it is too high, it may not produce any additional benefits but may produce adverse effects. This means that the dose should be in the optimum range, somewhere in between the 'No effect' and 'Harmful' effect regions.

This range which is called the 'therapeutic range', varies for different drugs, for example, the drug digitalis has a very narrow therapeutic range and hence their dose has to be calculated accurately to achieve the desired beneficial effect, while certain drugs like penicillin antibiotics have a very wide therapeutic range and hence their dosage can vary a lot (depending on the infection for which they are used) without changing their effects. This should explain why for certain drugs the dose is mentioned as one specific dose (for example, 200 mg twice daily) whereas for certain drugs, dose is mentioned as a range, for example, 200-500 mg twice daily.

The dose of a drug is dependent on factors such as age, weight and general health of the patient.

Age

The amount of medicine required by a patient is dependent on the age of the patient. Children weigh lesser than adults and hence they require lesser amount of drugs. Besides, their metabolic activity (action of liver on drugs to break down) is not as developed as the adults, nor is their excretory power (throwing out waste matter) as powerful. Hence the dose of a drug for children is very small as compared to adults. Children cannot simply be given a proportion of an adult dose as if they were small adults. Dosage has to be properly calculated considering the age and weight of the child.

The more accurate way of giving dosage of a drug for children would be to give it in the form: mg of drug/kg body weight of child. Giving the dose only on age basis would not be very accurate because two children of the same age may have quite a difference in their weights.

A premature baby (baby born before the full-term of nine months) may require higher doses of a drug initially. This is because the amount of water present in the body of a premature baby is high compared to the normal full-term baby, and hence certain drugs are not as concentrated in the premature baby as in a full-term baby. Likewise, doses for older people have also to be adjusted initially. Elderly people are more prone to adverse effects because their liver and kidneys are not as efficient as those of a normal adult. Besides they are not able to comply with the treatment properly on their own and need special attention.

	Prescribing for Children: Is a Drug Required?
	This question may be difficult to answer, whether the drug is for specific treatment or for symptomatic relief. Drugs are often used in children for self-limiting conditions and for symptoms for which drug efficacy has not been established.
	Problem areas include: (a)   antibacterial drugs used for upper respiratory tract infections that are usually viral; (b)   the overuse of decongestants for upper respiratory tract congestion, causing unacceptable adverse effects; (c)   the use of drugs in diarrhoea; (d)   the use of oral anti-emetics for vomiting; (e)   the use of antipyretic agents for fever; (f)   tricylic antidepressant drugs used for nocturnal bed-wetting (these account for three quarters of deaths in children due to accidental poisoning); (g)   the sedation of sleepless children or those falsely labelled hyperactive; (h)   the use of spasmolytics in abdominal pain; (i)   the use of drugs to increase appetite; and (j)   the use of "prophylactic" immunoglobulins for small children with frequent upper respiratory tract infections.
	In sum, these areas of drug use account for about 70 per cent of all medicines taken by children, and therefore as much as two thirds of all drugs used by children may have little or no value. The medicalization of some presenting problems and the inappropriate use of drugs for other conditions may have important consequences for future prescribing in terms of parents' demand for and expectation of drug therapy. The psychological and social consequences for the child given drugs in this way are not known, but children may tend to grow up believing that drugs are the solution to many of life's problems.
	Source : Drugs for Children. WHO Regional Office for Europe, Copenhagen, 1987