Copied from my previous answer, here:
There is a responsum from Rav Moshe about a patient refusing medical care, in the fifth volume of Zomet's halachic journal, תחומין. Here is a very loose translation/summary of what he writes there.
When a patient refuses to take (life-saving) medicine, it depends on
why he is refusing. If he still believes that he could be healed by this doctor, but is reluctant to do something which is painful for
him, which is a מעשה שטות ומעשה תינוקות, a juvenile/stupid thing to
do, then we force the patient to take the medicine, as much as we are
However, if he does not think that this doctor will be
successful at healing him, then we have to find doctors that he
believes will be effective at healing him. But if there are no doctors
who the patient thinks will be effective, or if it is impossible to
make the patient understand that what we are doing is for his benefit,
then we force him to take the medicine, so long as all the present
doctors feel that this is beneficial for him.
However, the only cases where we force a patient to pursue healing are
those where the patient will not become upset (שלא יתבעת מזה); for if
the patient should get upset, this can be be harmful, even
life-threatening. Thus, in those cases, it is better to do nothing.
Therefore, in every case where there is a patient that is unwilling to
undergo treatment, all of these variables need to be weighed
carefully, before forcing treatment on a patient; the only time
doctors should force treatment on a patient is when they do so לשם
שמים, with good intent.
Additionally, if there is any amount of סכנה, danger, in the pursuit
of this treatment, even if the doctors usually administer this
treatment, because the illness is more dangerous, the patient cannot
be forced into treatment.
With regards to administering a treatment that is dangerous, but the
danger of the illness is still greater (i.e., in general, and not in a
case where the patient is refusing treatment) -- one should not always
administer this treatment, because even if the treatment was proven to
be safe for healthy people, there isn't necessarily proof that it's
safe for a very ill patient. A potentially dangerous treatment should
never be administered, unless there is proof that more than half of the people who are sick with this illness, to the degree to which
this patient is sick, have recovered from their illness.
This calculation must be done carefully, because it is hard, even for very
great doctors, to calculate.
So the halacha in this case, according to Rav Moshe, is dependent on many factors -- the mindset of the patient, the chance of recovery, and the danger of the illness and the treatment. As with all halacha questions on Mi Yodeya, it's best to ask a knowledgeable rabbi for a final ruling.