Do Muslims Have to Seek Medical Treatment?
Question

If a person was dying from a terminal illness, and treatment offered was unlikely to benefit (although a remote possibility that it could help), does the person have to take the treatment?  As the treatment has many bad side-effects, and the person may not want to suffer them?  In general, does a Muslim have to take medicine for illnesses, or is it optional?
Praise be to Allah.Generally speaking, medical treatment or seeking a cure is allowed, because of the report of Abu’l-Darda (may Allah be pleased with him) who said: “The Messenger of Allah (peace and blessings of Allah be upon him) said: ‘Allah has sent down the disease and the cure, and has made for every disease the cure. So treat sickness, but do not use anything haram’” (Abu Dawud, 3376)
Usamah ibn Shurayk (may Allah be pleased with him) said: ‘The Bedouin said, “O Messenger of Allah, should we not treat sickness?” He said: “Treat sickness , for Allah has not created any disease except He has also created the cure, except for one disease.” They said, “O Messenger of Allah, what is it?” He said: “Old age.”’” (Al-Tirmidhi, 4/383, no. 1961) He said: This is a sahih hasan hadith. See also Sahih al-Jami’, 2930)
 The majority of scholars (Hanafi and Maliki) said that medical treatment is permitted.  The Shafi`is, and al-Qadi, Ibn ‘Aqil and Ibn al-Jawzi among the Hanbalis, said that medical treatment is recommended, because of the hadith “Allah has sent down the disease and the cure, and has made for every disease the cure. So treat sickness, but do not use anything haram,” and other hadiths which contain instructions to seek cures.  
They said: the fact that the Prophet (peace and blessings of Allah be upon him) used cupping and other kinds of treatment indicates that medical treatment is permitted.  
For the Shafi`is, treatment is mustahabb when there is no certainty that it will be beneficial , but when treatment is certain to be beneficial (such as putting a dressing on a wound), then it is obligatory (an example would be blood transfusions in certain cases).
(See Hashiyat Ibn ‘Abidin, 5/215, 249; al-Hidayah Takmilat Fath al-Qadir, 8/134; al-Fawakih al-Dawani, 2/440; Rawdah al-Talibin, 2/96; Kashshaf al-Qina’, 2/76; al-Insaf, 2/463; al-Adab al-Shar’iyyah, 2/359ff, Hashiyat al-Jumal, 2/134)
 Concerning the sahih hadiths that speak about medical treatment , Ibn al-Qayyim said: 
“This does not contradict tawakkul (putting one’s trust in Allah), just as warding off hunger, thirst, heat and cold does not contradict tawakkul.  The essence of tawakkul is not complete without resorting to the means which Allah has set out in order for us to achieve results both according to His decree (qadr) and His laws (Shar`). 
Not using these means is contrary to tawakkul: it goes against and undermines the command and wisdom of Allah, although the one who neglects the means may think that this makes his tawakkul stronger. Ignoring the means is a sign of helplessness that goes against the true essence of tawakkul, which is that the heart relies on Allah to bring the slave whatever will benefit him in this world and the next, and to protect him from whatever may harm him in this world and the next. But along with this reliance, it is essential to take the appropriate means, otherwise he will be going against the wisdom and command of Allah.  Helplessness should not be taken as a sign of tawakkul, nor should tawakkul make a person helpless.” (Zad al-Ma’ad, 4/15. See al-Mawsu’ah al-Fiqhiyyah, 11/116)
In summary, therefore, seeking a treatment or cure is not obligatory according to the scholars, unless – according to some – it will definitely be of benefit.  Since in the situation described in the question there is no certainty that treatment will be of benefit, and indeed it is likely to cause suffering to the patient, then there is nothing at all wrong with not giving the treatment.  
The patient should not forget to put his trust in Allah and seek refuge in Him, for the gates of Heaven are open to those who call on Allah.  He may also seek treatment (ruqya) by reciting Quran , such as reading al-Fatihah, al-Falaq and al-Nas over himself.  This will benefit him psychologically and physically, as well as bringing him reward.  Allah is the Healer and there is no healer but He.
And Allah knows best.