Unwanted Interactions With Antihypertensive Drugs

Specific interactions are described in the accounts of individual drugs. The following are general examples for this diverse group of drugs.

Alcohol intake is the commonest contributing factor, or even cause of hypertension, and should always be considered as a cause of erratic or failed

36 The Magpie Trial Collaborative Group 2002 Lancet 359: 1877-1890.

37 The Eclampsia Trial Collaborative Group 1995 Which anticonvulsant for women with eclampsia? Evidence from the Collaborative Eclampsia Trial. Lancet 345:1455-1463.

responses to treatment (measurement of the y-glutamyl transpeptidase and red cell mean corpuscular volume may be useful).

Prostaglandin synthesis. Nonsteroidal anti-inflammatory drugs (NSAIDs), e.g. indomethacin, attenuate the antihypertensive effect of ß-adrenoceptor blockers and of diuretics, perhaps by inhibiting the synthesis of vasodilator renal prostaglandins. This effect can also be important when a diuretic is used for severe left ventricular failure.

Enzyme inhibition. Ciprofloxacin and Cimetidine inhibit hepatic metabolism of lipid-soluble ß-adrenoceptor blockers, e.g. metoprolol, labetalol, propranolol, increasing their effect. Methyldopa plus an MAO inhibitor may cause excitement and hallucinations.

Pharmacological antagonism. Sympathomimetics, e.g. amphetamine, phentolamine (present in anorectics and cold and cough remedies) may lead to loss of antihypertensive effect, and indeed to a hypertensive reaction when taken by a patient already on a ß-adrenoceptor blocker, due to unopposed a-adrenergic stimulation.

Surgical anaesthesia may lead to a brisk fall in blood pressure in patients taking antihypertensives. Antihypertensive therapy should not be routinely altered before surgery, although it obviously can complicate care both during and after the operation. Anaesthetists must be informed.

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