Phenytoin for the prevention of motion sickness

A 43-year-old woman sought advice about the treatment of her sea-sickness. A sea-kayaking enthusiast, she suffers disturbing motion sickness in big seas. The usual remedies have either been ineffective or made her drowsy. A scuba-diving instructor recommended that she try taking phenytoin the night before going kayaking, and she wonders whether there is any evidence to support this recommendation.

The primary search question was "Is phenytoin effective in the prevention of motion sickness?" The ideal studies to answer this question would be randomised controlled trials comparing phenytoin with placebo or comparing phenytoin with other conventional therapies.

I searched two online databases, MEDLINE and SUMSearch, combining the terms "motion sickness" and "phenytoin" and limiting the search to studies published in English. The MeSH term "motion sickness" covers a range of conditions, including sea-sickness and travel-sickness. I also searched the websites for two diving-related organisations: Divers Alert Network and Diving Medicine Online.

Five original research articles on the use of phenytoin for motion sickness were identified. Diving Medicine Online had a review article on motion sickness that mentioned phenytoin.

There is evidence that phenytoin is effective in preventing motion sickness. The approach used in most of the studies — that of rapidly loading subjects up to anticonvulsant levels of phenytoin in the preceding 24 hours and monitoring plasma levels — is hardly applicable for practical purposes. The generalisability of these studies is also questionable, as all were small and used healthy adult males. Taking a 200 mg dose of phenytoin four hours before exposure is practical, and the results are promising but not conclusive. Further research is required to determine the lowest effective dose. Short-term use of low doses is unlikely to produce serious problems. However, the fact that phenytoin is known to interact with some commonly used medications should be taken into account if it is to be prescribed for preventing sea-sickness.

The woman elected to try a single 200 mg dose of phenytoin four hours before going kayaking. This was considered the lowest dose that might prove effective, based on the available evidence. To test for side effects, the woman accepted my advice to try the first tablet on a day when she wouldn't be engaging in hazardous activity. (I can't report whether she was satisfied with the result, as I haven't heard from her since — that's the trouble with fit, healthy people!)