Nobody knows, however, who among those who have had an adverse reaction will be able to tolerate the drug on a second ground, and the long-term effects are not known.

Furthermore, trials to date have been carried out on patients with mild to moderate AD who were in moderately good health. Elderly demented patients with multiple medical illnesses may be more likely to develop side effects (JAMA, November 11, 1992). It has also been suggested that patients with more severe disease do not benefit from tacrine therapy (New England Journal of Medicine, 1986:315; 214-5).

But are the known risks involved in tacrine treatment worth it? Few can even agree about that. An editorial in the BMJ (April 2, 1994) reported that "modest" beneficial effects of tacrine treatment had been reported in between 3 per cent and 50 per cent of trial subjects, with only a very few patients improving greatly. Another study (JAMA, April 6, 1994) gave a one-in-three chance of improvement with tacrine.

Dr Margaret Winkler, a senior editor of JAMA, warned that even if a patient does respond positively to tacrine "the decline associated with AD continues, and since the drug's action is temporary, if administration of the drug is stopped, the patient's cognition will return to the level expected if the drug had never been administered" (JAMA, April 6, 1994).