Why do some medication (prescribed ones) mention they may make happen counterweight gain?

I just looked info on neurontin & possible weightiness gain is mentionedWhy do some medication (prescribed ones) mention they may make happen counterweight gain?
they usually cause one of two things.
eiether your body starts recounting you it's "not getting something" and you eat more food later you would usually. And if you don't compensate by working-out you gain weight
or your everyday trigger that tells you to stop intake when you are full is slowed.. or even barely existant when you lift the medication.
They cause you to go and get hungry which in turn make you gain wieght.
They have to disclose adjectives side affects!!