Only some drugs need an accurate weight, most (especially the ones doled out by GPs) don't. However - some doctors like to keep check of peoples weight, smoking habits, drinking habits, etc in order to (a) Provide decent advice, and (b) sometimes they get money for reporting the same to the government, As for a reason to not provide treatment...well, no, not unless the person was so obese the treatment either wouldn't work, or would have a reduced chance of working. For example heart transplants. But for a hangnail - no. Did that answer your question?