Wa `alaykum as-salam wa rahmatullah.

 

It depends on the case and status of the patient. If it seems necessary for him to know his disease to help on caring about himself and avoiding certain foods or to take certain medicines, it is obligatory to do so. This is to keep him safe.

 

But if it does not make a difference, it is recommended not to tell him the truth about his disease. The physician must make his patients hopeful about the future. It was narrated from Abu Sa`eed Al-Khudri that the Messenger of Allah (peace be upon him) said: “When you enter upon one who is sick, cheer him up and give him hope of a long life, for that does not change anything (of the Divine Decree), but it will cheer the heart of the one who is sick.” (Ibn Majah and Abu Dawud, it is however graded as weak hadith)