I may be wrong but I heard that Islam forbid to take poison. Chemotherapy is kind of a poison. Then is it sinful to apply chemotherapy for cancer patients?
The hadiths, in Sahih Al-Bukhari » Book of Medicine » Hadith 5778 and other books, that prohibit taking poison are aimed at the intention of one trying to commit suicide or to intentionally harm oneself. They do not apply to the case of chemotherapy when taken as a possible treatment to cancer or any other malady.
In Islam, our belief is that remission comes only from Allah ﷻ. Prophet Ibrahim ﷺ, when debating with his polytheist tribe, said:
وَإِذَا مَرِضْتُ فَهُوَ يَشْفِينِ
And when I am ill, it is He who cures me.
Prophet Muhammad ﷺ further explains that there is no cure but Allah's cure:
عَنْ عَائِشَةَ أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا عَادَ مَرِيضًا يَقُولُ: أَذْهِبِ الْبَاسَ رَبَّ النَّاسِ اشْفِهِ أَنْتَ الشَّافِي لاَ شِفَاءَ إِلاَّ شِفَاؤُكَ شِفَاءً لاَ يُغَادِرُ سَقَمًا
'A'isha reported Allah's Messenger ﷺ as saying: When Allah's Messenger ﷺ visited the sick he would say: "Lord of the people. remove the disease, cure him, for Thou art the great Curer, there is no cure but through Thine healing Power, which leaves nothing of the disease."
Having said that, as Muslims, we are ordered to do our part (our homework, so to speak). According to our belief, medication is a cause that leads to a desired effect (cure). In Sahih Muslim » The Book of Greetings » Hadith 2204, Prophet Muhammad ﷺ teaches us that for every malady, there is a remedy. We may know or not know that remedy, but Allah has provided a remedy for every malady.
There is a weak (da'if) hadith in Sunan Abi Dawud » Book of Medicine » Hadith 3874 that one should not use a medicine that is unlawful. Weak hadiths may be used for a multitude of purposes, but the majority of scholars do not derive rulings of permissibility (or lack of) based on a single weak hadith.
Chemotherapy is not poison per se. It is a medically-approved treatment of cancer through a number of chemicals that may be poisonous in nature, but are not administered with the intention of killing or inflicting harm on the patient. This is similar to spraying plants with insecticides; the intention is not to kill the plants by use of poisonous materials in the insecticide.
Exercising common sense in the presence of strong side effects (as is the case with most chemotherapy regimens) is key in this matter. As per the answer to the question "Harmful" implies "haram" — what trade-offs exist?, harm must be warded off as much as possible, but when there is a potential for both benefit and harm (as in the case of chemotherapy), then:
- If the risk outweighs the benefit, one has to avoid chemotherapy.
- If a condition is present temporarily that causes chemotherapy to be more risky (e.g., deteriorating health conditions in between cycles or regimens), one has to wait for the condition to change.
- If the benefit outweighs the risk, one is permitted to take chemotherapy.
It is also worth noting that if a Muslim doctor prescribes medicine to a Muslim patient, this would be considered an expert opinion on the matter:
وَمَا أَرْسَلْنَا مِن قَبْلِكَ إِلَّا رِجَالًا نُّوحِي إِلَيْهِمْ ۚ فَاسْأَلُوا أَهْلَ الذِّكْرِ إِن كُنتُمْ لَا تَعْلَمُونَ
And We sent not before you except men to whom We revealed [Our message]. So ask the people of the message if you do not know.
While the specificity of this verse is related to a particular situation, the general rule derived from it is that the opinion of people of knowledge or experts has its value in weighing alternatives in matters where difference of opinions exist.