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Sure you will gain weight and muscle mass quickly but the side effects can be nasty.
As many of the weight gain supplements are not approved by FDA.
It is better to avoid weight gain supplements.
Because they usually contain steroids
They work in the short run but they usually mess up the body.
They Primarily affects the liver
They even reduce the bone mineral density.
They may contain anabolic steroids which can decrease your sexual desire and libido if you are male.
In female, it can cause increased growth of body hair, facial hair, deepening of voice.
So your voice may become like a man.
It also interferes with the menstrual cycle and in the worst your ovaries atrophy
, rendering you sterile.
Better to have good exercise and healthy food as prescribed by your Physician and Dietician
Hope this helps.