All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Allaah The Almighty created a remedy for every disease and made medical treatment a means for cure, by the permission of Allaah The Almighty. Treatment by itself does not cure the disease or accelerate healing without the permission and predestination of Allaah The Almighty.
A person does not know what Allaah The Almighty has predestined for them, that is, it may be predestined for him to recover through medical treatment, or without any medicine, or his recovery may be delayed if he does not take medicine.
As a believer does not know what Allaah The Almighty has predestined for him, it is better for him to put his complete trust in Allaah The Almighty and believe that He is the only One Who can cure him and that all things occur according to His Will.
Also, a person should supplicate Allaah The Almighty and ask for His help before searching for a remedy. Then he should take the medicine according to the advice of the Prophet who commanded us to make use of medical treatment. [Ahmad] Indeed, the Prophet, sallallaahu ‘alayhi wa sallam, said Allaah The Almighty did not make any disease without appointing a remedy for it, with the exception of one disease: old age. [Abu Daawood and At-Tirmithi]
Allaah Knows best.