Yes, lithium can impact weight gain. However, the truth is that at the age of 21, and being female,your daughter can maintain or even lose weight while being on the drug. True, it is more of a challenge, but many of my younger clients who take lithium are thin. They must watch their diets carefully and exercise, and some receive support through weight watchers and similar, but to encourage your daughter, yes...this is more than possible.
There are many drugs that treat this issue, but I suspect your daughter has been through many of them. Olanzapine (Zyprexa), Aripiprazole (Abilify), Risperidone (Risperdal)Ziprasidone (Geodon) and Clozapine (Clorazil) are in this group as are some of the antidepressants. But, lithium is still a gold standard for many, and this naturally occurring salt is nothing short of miraculous in stabilizing moods in many cases.
No, it is not easy to maintain or lose weight while on lithium, but it is more than possible with diet, exercise and support. Some also (with physician's approval) find that Omega 3 Fatty Acids help with weight gain and mood stabilization. It is not completely researched, but It is worth exploring. Steven