There is no harm to use weight control drugs as long as there is no harmful effect on one’s health. The kind of drug used should not defy the precept of Islamic jurisprudence.
Dr. Husam al-Din Ibn Musa `Afana, professor of the Principles of Islamic Jurisprudence at the University of Jerusalem, states: There is no harm in taking weight-reduction drugs on the condition that it does not have any harmful effects. Above all, the drug taken should be Islamically lawful. Islam encourages its followers to eat moderately and avoid excess. Obesity usually appears as a result of overeating. The Prophet (peace and blessings be upon him) is reported to have said, “The human being can not overfill his stomach without harming himself. It is the worse kind of container. He should only eat enough to keep himself healthy and active. Therefore he should keep a third of his stomach for his intake of food, a third for his intake of fluids and a third for his breath.” (Ahmad At-Tirmidhi)
The Prophet (peace and blessings be upon him) met a very fat person with a very big belly. He then pointed to the man’s belly and said, “If this was not there it would have been better for you.” (Ahmad and At-Tabarani)
While commenting on the above Hadith, a well-known scholar, Sheikh As-Sa`ati, reiterated that if the largeness was in another part of the body other than the stomach, it would have been better. This is because the bigger the belly the heavier the man, and the more apt he is to ailments. Overeating is disapproved of as the Prophet recommended to eat and drink moderately because doing so is healthier for the human body.