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In my opinion, chemotherapy harms the body and does more damage than benefit. It does little to improve you and rather ends up destroying your entire immune system, causing you to become further physically weak.
Would it be permissible for one who holds this opinion to turn down chemotherapy, provided they have cancer?
First of all when it comes to the fiqh ruling on treatment scholars hold different opinions:
The source of all these interpretations is the hadith that was compiled by many scholars such as imam Ahmad and al-Hakim in his al-Mustarak and the authors of the Sunan:
"Some Bedouins asked: 'O Messenger of Allah (s.a.w) shall we treat (our ill)?' He said: 'Yes, O worshipers of Allah! Use remedies. For indeed Allah did not make a disease but He made a cure for it' - or - 'a remedy. Except for one disease.' They said: 'O Messenger of Allah (s.a.w)! What is it?' He said: 'Old age.'" (Source Jami' at-Tirmidhi)
When it comes to the question :"Is stopping or turning down a treatment equal to suicide?" The answer is no, according to this fatwa -in Arabic- on islamweb #27266, but if the person knows of the benefit or healing of a treatment or medicament and is able to take it, this person would commit a sin.
In osol al-Fiqh doubt is the opposite of certainty and if there's a hint of doubt as you suggest about chemo therapy most scholars would say he is allowed to turn it down.
islamweb even has two concrete fatwa on cancer #179611 and #64214 -both in Arabic-. The main opinion held by the muftis there is that treatment is not obligatory and in case of it being known to be helpful it is better to take/follow it.
