Is a Muslim obligated to seek treatment in a case of advanced terminal illness, where the treatment has side effects and offers little hope of cure, or is seeking treatment generally optional?

Generally, seeking medical treatment is permissible. It is considered recommended by the Shafi'is, Al-Qadi, Ibn 'Aqil, and Ibn Al-Jawzi from the Hanbalis. It is deemed permissible by the majority of scholars (Hanafis and Malikis). It becomes obligatory according to the Shafi'is if its benefit is definitively established.

Seeking medical treatment does not contradict reliance on Allah (Tawakkul); rather, the true essence of Tawhid is only realized through engaging with means. In summary, seeking medical treatment is not obligatory unless its benefit is definitively established according to some scholars. If its benefit is not definitively established, and the patient suffers psychological distress from it, then there is no harm in abandoning it. In such a case, one should rely on Allah, supplicate, and treat oneself with the Qur'an (ruqyah).