Transdermal hyoscine (Scopolamine). A preliminary review of its pharmacodynamic properties and therapeutic efficacy.

Hyoscine (scopolamine) is a competitive inhibitor of the muscarinic receptors of acetylcholine and it has been shown to be one of the most effective agents for preventing motion sickness. However, a relatively high incidence of side effects and a short duration of action has restricted the usefulness of this agent when administered orally or parenterally, and to counter this a novel transdermal preparation of hyoscine has been developed. Pharmacokinetic studies indicate that this new method for administering hyoscine controls the absorption process and the rate of drug entry into the systemic circulation over an extended period (72 hours), providing a means of delivery which is similar to a slow intravenous infusion. However, recent evidence suggests that the response to transdermal hyoscine treatment is variable and this may reflect pharmacokinetic differences between individuals. Controlled therapeutic trials have indicated that a single transdermal hyoscine patch is significantly superior to placebo and oral meclozine (meclizine) in preventing motion sickness. Trials comparing transdermal hyoscine with oral dimenhydrinate have failed to establish any significant differences in efficacy between the 2 drugs in small numbers of subjects, although there was always a more favourable trend towards the transdermal system. In patients with acute vertigo, transdermal hyoscine and oral meclozine were equally efficacious and both were significantly better than placebo in reducing the number of attacks of vertigo. Although transdermal hyoscine has been associated with a lower incidence of side effects than orally or parenterally administered hyoscine hydrobromide, adverse systemic effects have still been frequently reported. Most commonly cited have been dry mouth, drowsiness and impairment of ocular accommodation, including blurred vision and mydriasis (some ocular effects reported may be due to finger-to-eye contamination). Adverse central nervous system (CNS) effects, difficulty in urinating, rashes and erythema have been reported only occasionally. Thus, preliminary evidence suggests transdermal hyoscine may offer an effective and conveniently administered alternative for the prevention of motion-induced nausea and vomiting in certain situations. However, the duration of its clinical effectiveness, and its relative efficacy and tolerability compared with other agents needs to be confirmed in a few additional well-designed studies.