Not necessarily, but it wold depend what your problem is on why you can't gain weight if that's a problem for you. I the reson you can't gain and is b/c of a defincency, then it could help you gain weight, just depends what the issue is w/ your body.
They should not cause you to gain unnatural weight. But like the previous poster said, if it's correcting a deficiency in you that's been preventing your body from gaining weight, then your body will become healthier (ie. begin gaining that weight back)