Rav Nosson Tzvi Finkel [EDIT: was rumored to have] refused to take medication for his Parkinson's [EDIT: supposedly] because he felt it reduced his mental acuity and feared it would interfere with/cause him to forget his learning.
A friend with Parkinson's recently told me that he had heard this rumor (which dates back over a decade; I heard it for the first time in 1997), and that he strenuously objects to the publicizing of the rumor. In his opinion, there are certain drugs associated with Parkinson's treatment that one is required to take, and by promoting this rumor as a means to inspire people, he feels it does an injustice to R' Finkel ZT"L's memory.
While my friend cannot refute the rumor (and I cannot substantiate it), he surmised that R' Finkel may have refused a particular drug, perhaps very long ago, which may have led to this story/rumor spreading.
I can attest as an eye-witness, having met R' Finkel twice, that he did exhibit severe Parkinson's symptoms that made it look like he did not take treatment, or at least not the treatments commonly used to reduce the spasms associated with the disease.