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Principles of clinical pharmacology and drug therapy 

Principles of clinical pharmacology and drug therapy

Principles of clinical pharmacology and drug therapy

Kevin O’Shaughnessy

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date: 27 March 2017

The role of clinical pharmacology is to provide the scientific basis for rational prescribing: ‘patients may recover in spite of drugs … or because of them’ (Gaddum). This sums up the dilemma facing any doctor who prescribes a drug to a patient: it should certainly be the doctor’s explicit intention to do the patient some good, but the drug may actually harm the patient, and on rare occasions it can even kill them.

Principles of clinical pharmacology

Drug therapy can be considered under four headings: (1) pharmaceutical—is the drug getting from the formulation into the patient?; (2) pharmacokinetic—how does the drug dose, formulation, frequency, and route of administration affect the drug concentration in the body, and the way that this concentration changes with time?; (3) pharmacodynamic—how does a drug produce its pharmacological effects?; and (4) therapeutic—is the pharmacological effect being translated into a therapeutic effect?

Adverse drug reactions

Definition and causes—an adverse drug reaction (ADR) can be defined as an unwanted or harmful reaction experienced following administration of a drug, or combination of drugs, under normal conditions of use that is suspected of being related to the drug (or combination). These can be (1) dose related—usually due to an exaggeration of a known pharmacological effect of the drug; or (2) non-dose-related—often caused by immunological or pharmacogenetic mechanisms.

Clinical importance—ADRs are responsible for 1 to 4% of acute hospital admissions, affect 5 to 20% of inpatients at some time during their admission, and are responsible for up to 3% of inpatient deaths. Pharmacovigilance is the subspecialty of clinical pharmacology devoted to the detection and evaluation of ADRs.

Drug interactions

A drug interaction occurs when the effects of one drug are altered by the effects of another drug, usually resulting in an ADR. Drugs likely to precipitate interactions often (1) are highly protein bound, e.g. aspirin and sulphonamides; or (2) induce drug metabolism, e.g. phenytoin, carbamazepine and rifampicin; or (3) inhibit drug metabolism, e.g. cimetidine, metronidazole, and triazole antifungals. The drugs most likely to be affected by drug–drug interactions are those with a steep dose–response curve and a low therapeutic index.

A rational basis for prescribing

This requires that a drug’s therapeutic potential is maximized and its side effects minimized. A series of checks and balances for the process of drug prescription is needed. A question checklist that can usefully be applied is as follows: (1) does the patient need a drug at all, that is, do the risks outweigh its benefits? (2) are the benefits of the drug well established, preferably by randomized controlled trials? (3) what drug action is being sought, and what class of drug can best provide it? (4) what is the most appropriate drug in that class, and in what formulation? (5) what is the appropriate dose, and how frequently or in what circumstances should it be taken? (6) for how long should the drug be prescribed?—is it as a single course or for indefinite use? (7) will this drug interact with other drugs the patient is taking? (8) can this drug replace other drugs the patient is taking? (9) what does the patient need to understand about this drug?—and who will communicate this, and how? (10) will it be necessary to review the prescription of this drug?—and if so, when, how, and by whom? (11) does the patient need anything else to derive the most benefit from this drug?

Practical prescribing

Guidelines and formularies—the question checklist above is not completed before every drug is prescribed, many of the questions being addressed by using appropriate therapeutic guidelines and formularies. Guidelines provide prerehearsed decision paths for many of the issues raised in the checklist, whilst formularies specifically tackle the question of which drug to prescribe (from within a therapeutic class).

The patient’s drug history—it is essential to obtain a thorough drug history from the patient before prescribing. Key information that should be obtained includes details of: (1) all the medicines currently being prescribed, including their doses; (2) any previous medical treatments; (3) any ‘alternative’ treatments, e.g. herbal and homeopathic remedies; (4) any self-prescribed medicines; (5) any history of allergy or adverse reactions to drugs.

Prescribing for the particular patient—guidelines, formularies and other prescribing aids are not a substitute for an intelligent clinical approach. The prescriber needs to establish what the patient’s experience and expectations of drug therapy are, and the patient needs to know the likely consequences—both good and bad—of taking any drug that is prescribed. This dialogue is important, since it will often decide whether the patient actually takes the drug as prescribed. Patient compliance is a key variable in the prescribing process, and one in which the doctor often has least control.

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