What is the ruling on one who causes their own health to deteriorate by using medication not prescribed by a doctor? Is this considered a matter of divine decree (Qada' and Qadar), and will they be rewarded for this affliction if they endure it with patience?

It is not permissible for a Muslim to cause harm to themselves or to others, whether it be to their religion, body, mind, or wealth, because the Prophet, peace and blessings be upon him, said: "There should be no harm nor reciprocal harm." And everything that occurs in the universe is by the decree and preordainment of Allah the Almighty, for He says: "Indeed, We have created all things with a [precise] measure." This person who has caused their health to deteriorate must seek treatment, for seeking treatment is among the legitimate means that the Prophet, peace and blessings be upon him, guided us to when he said: "Seek medical treatment, for Allah has not sent down any disease but that He has also sent down its cure, except for one disease, which is old age." And whoever patiently endures what afflicts them of illnesses, out of faith and hope for reward, will be rewarded for it, based on his saying, peace and blessings be upon him: "Wondrous is the affair of the believer, for all his affairs are good. If prosperity befalls him, he gives thanks, and that is good for him. And if adversity befalls him, he perseveres patiently, and that is good for him," and his saying: "No fatigue, nor disease, nor sorrow, nor sadness, nor hurt, nor distress befalls a Muslim, even a thorn that pricks him, but Allah expiates some of his sins thereby."