Choosing the Right Dosage. Controlled Drugs.
Notes on dosage
In this dictionary of drugs, the doses, unless otherwise stated, are average daily doses for adults, and are normally given as divided doses at suitable intervals. An indication is given of those few drugs that are taken as a single daily dose, or where
the dose is based on the surface area of the body expressed as square metres (ml).
When a dose range is indicated, a small initial dose is often followed by gradually increasing doses according to need and response, but with long-acting drugs care must be taken to avoid excessive dosage, as such drugs may then accumulate in the body and have toxic side-effects. It should be noted that the side-effects of a drug may prevent the administration of an optimum therapeutic dose, and a recommended dose is often a compromise between activity and toxicity. The gap is sometimes referred to as the ‘therapeutic window’, and the wider the gap the safer the drug.
Dosage is also influenced by the route of administration, as oral doses are normally larger than those given by intramuscular injection, and the latter are in turn larger than those given intravenously. The weight, age and sex of a patient may also have to be taken into account when assessing dosage. Renal and hepatic
-impairment may require an adjustment of dose, whereas if tolerance to a drug
develops, a larger dose may be required to evoke an adequate response. Dosage
also requires particular care during pregnancy if toxic effects on the fetus are to be avoided and, ideally, no drug should be given in pregnancy unless the need outweighs the risks.
Doses for infants also present problems. Their drug-detoxifying enzyme
systems and renal excretory capabilities are not fully developed although,
paradoxically, they have higher metabolic rates than adults, and so may sometimes require an apparently relatively high dose of a drug.
Doses for the elderly also frequently require modification, as their reduced renal efficiency may cause accumulation of the drug and increase susceptibility to toxic effects, unless reduced doses are given. Elderly patients are, in any case, more susceptible to nephrotic drugs generally. Hepatic disease may also increase drug toxicity as liver enzymes play a major part in drug metabolism, and severe liver disease may give rise to a drug toxicity that might not otherwise occur. Hyperlipidaemic drugs, fusidic acid and some antifungal agents of the ketoconazole type are examples of drugs best avoided in hepatic disease. Multiple therapy may also lead to an increased and not always appreciated incidence of drug toxicity. For further information on these important aspects of drug dosage, a book for nurses on drugs and pharmacology should be consulted.
Controlled drugs
Drugs likely to cause dependence and misuse are referred to as ‘controlled drugs’ (CDs), as they are subject to the strict prescription requirements of the Misuse of Drugs Act, 1971, and are distinguished in this book by a dagger t. They include opium, morphine, heroin (diamorphine), pethidine and other synthetic, potent analgesics/narcotics, dihydrocodeine injection, barbiturates (except intravenous anaesthetics) and amphetamines. Controlled drugs can be supplied only on receipt of a hand-written prescription from the prescriber, giving full details of the patient, the drug, the form and strength of the prescription to be dispensed, and the total amount to be supplied expressed in both words and figures. Certain weak preparations of some controlled drugs, such as Kaolin and Morphine Mixture for diarrhoea, with which misuse is unlikely, are exempt from control, and can be obtained without prescription.