The principle for the National Health Service is easy. The practice is a great deal more difficult. Around the world, health systems have reacted in sharply differing ways to the arrival of Viagra, the anti-impotence pill. In the UK, the challenge for health ministers is to decide a set of principles which can be applied not just to Viagra but to other lifestyle drugs which are just over the medical horizon - obesity drugs and memory enhancers, for example. (editorial)