Most of us as being patient will get annoyed, why every time doctor prescribes me lot of medicine including vitamins. Is that really necessary? Some may think it as an economic burden. Why vitamin are given with antibiotics?
Vitamins might be necessary when antibiotics are given because antibiotics may eliminate or at least reduce the bacterial population of the intestine, thereby curtailing the supply of vitamins synthesized by these bacteria. Intestinal bacteria synthesize vitamins including biotin, vitamin B12, folic acid, and thiamine.
However, not all antibiotics reach the large intestine in amounts sufﬁcient to alter the bacterial population signiﬁcantly, and there is no vitamin for which the bacterial source is generally critical (except perhaps vitamin B in strict vegetarians). But the practice of medicine can be quite different from theory. If there is the slightest justiﬁcation for giving relatively harmless substances such as vitamins, most physicians would like to give them. An additional justiﬁcation is that vitamin requirement may be increased in these patients due to stress of the infection for which antibiotics are being given.
Source: RL Bijlani; Understanding Medical Physiology 4th edition