Apparently the doctor advised my friends to get 250 mg instead of the normal 500 and take one daily as a preventative measure
It is this type of irresponsible use of anti-biotics for every minor ailment which is leading to the appearance of drug resistant strains of assorted bugs - most visibly in the media recently is MRSA in UK hospitals...
Don't forget, your own body has a very effective built in immune system - keep it healthy and it'll look after you rather well without having to resort to the use of preventative drugs...