Doctor, doctor: Melatonin and Alzheimer's

My sister in America can buy melatonin without prescription to help her sleep. I'm told that here I'd need a prescription, and that there are restrictions on its use. Why is this? My sister says it works for her.
Until recently, melatonin hasn't been licensed in the UK, but one form of it has now been given the go-ahead. It can be prescribed for only three weeks, for patients aged 55 and older with "primary insomnia". That's because we see it as a drug and the Americans, apparently, look on it as a supplement. The European Medicines Agency concluded from trials so far that it has a small effect in a relatively small fraction of patients. It is probably about as effective as modern sleeping tablets (except that comparative trials haven't been done), with no obvious advantages except it may cause less dependence and fewer withdrawal effects than others. There are several caveats: it shouldn't be given to patients with liver disease, or who are pregnant or breastfeeding, and it interacts with several prescription drugs. If you smoke, it will have far less effect, and you can't combine it with alcohol.

My grandfather and an uncle died from Alzheimer's disease. What can I do to find out my chances? Would genetic testing help?
Probably not. For example, a negative test result for the genes linked statistically with a higher than normal risk of Alzheimer's wouldn't prevent you developing dementia in later years. And being given a positive result doesn't mean you'll develop Alzheimer's – it only slightly raises your chances. In a recent US trial, 162 non-anxious and non-depressed relatives of Alzheimer's patients were tested for the variation of apolipoprotein E, possession of which is linked to risk of the disease. Two-thirds were told their results and one-third were not (they were all volunteers and accepted this beforehand). There was little difference in mood afterwards, regardless of the result or of being kept in the dark, except that those given negative results were marginally happier than those given positive ones. As we don't yet know how to prevent Alzheimer's, or how to reverse its effects once established, there doesn't seem any point in having the test.

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