Do multivitamins help gain weight?

Probably NOT. Multivitamins aid in metabolism but will not cause weight gain per se. Weight gain is governed by extra calories taken in minus calories needed for the body to function. However, one medical condition =hyperthyroidism causes a 'hyper-metabolic state' causing weight loss and other symptoms. Thyroid blood tests should be checked if unexplained weight gain/loss is noted.