I suspect that Wendy is correct in why it was prescribed, but to answer your generic question, at least in the U.S., a physician can prescribe any medication for any purpose. If I remember correctly, this is called an 'off schedule' use or something like that. As the side effects of drugs become known, they aften get prescribed for the side effect. This is how a lot of drugs get accepted for other use. For instance, it's how Viagra came to be used for the purpose everyone knows about - that is NOT what it was developed and approved for. But the observed side-effects led to its being prescribed off-schedule.