All medications have side effects. Hives, constipation, dry mouth, and headaches are common side effects in a lot of drugs. But in some cases certain side effect combinations can indicate an allergic reaction instead of being a side effect. Be sure to read the specific warnings on the drug to be sure.
Overall, the entire suite of B-complex vitamins is essential for good hair and skin. In addition to Vitamin B12, vitamins B3 and B6 are also critical for hair and skin growth and appearance. A balanced B-complex vitamin supplement provides the extra boost your hair and skin may need to look great.
Specific vitamin and mineral deficiencies like iron or vitamin B12 may be diagnosed by blood tests and treated. Multiple vitamins, including biotin, have been promoted for hair growth, but solid scientific studies for many of these claims are lacking. While taking biotin and other supplements marketed for hair, skin, and nails probably won't worsen anything, it may also not necessarily help the situation. Therefore, advertised hair-regrowth supplements should be approached with mild caution.