It should be clear that Islam forbids what is harmful or impure. Therefore, using weight gaining products is permissible if they do not contain harmful and impure ingredients and these supplements are proven to be safe. We suggest that you make a medical consultation regarding proper and safe supplements or measures.
The prominent Muslim scholar, Sheikh Muhammad Iqbal Nadvi, Imam of Calgary Mosque, Alberta, Canada, and former Professor at King Saud University, Riyadh, Saudi Arabia, states: “There is no Islamic restriction to control the weight. The only thing you need to be careful about is that YOU DON’T USE ANYTHING HARAM OR HARMFUL TO YOUR BODY. This is based on the fact that Islam protects the mental and physical health and considers this as one of the top priorities. The best Qur’anic guideline for diet is: “Eat and drink: But waste not by excess, for Allah loves not the wasters” (Al-`Araf: 31).
Therefore, you are allowed to use these supplements while fulfilling the abovementioned conditions. By doing so, you will not be doing an act of altering the creation of Allah.”