These are the Rebbe's letters to individuals about their own situations. Throughout, the Rebbe instructed people to consult an expert doctor and follow his guidance; anyone facing a medical question should do the same.
Following medical advice is a Torah obligation. The Torah gives the doctor permission and a mandate to heal, and seeking treatment is not only permitted but required; one must follow the doctor's guidance even without fully understanding or agreeing with it, and even when one feels well (IK 1395, 3085, 3089, 3166, 3761, 5475). Being stringent against the doctor's instructions, including on diet, is not piety (IK 4725).
Caring for the body is itself serving Hashem. The body belongs to Hashem, and keeping it healthy is a mitzvah, not a waste of time or a lowering of one's spiritual level; it is what gives the strength to serve Hashem with joy and energy (IK 605, 2209, 2383, 2644, 3467, 3646, 4962, 5393, 6551, 6574). Follow the advice with joy and without overthinking (IK 2282).
Choosing and working with doctors. Seek real expertise, and in complex cases a general doctor who can coordinate the care (IK 1266, 7995). What matters most in a practitioner is practical skill (IK 2094). One may consult a non-Jewish doctor; in medical matters what counts is expertise (IK 3512). Be open to newer treatments, but the decision belongs to the physician (IK 3627). Where old sources caution against certain procedures, such as tooth extraction, follow current medical advice (IK 2572, 4349). When an expert deems a procedure necessary, follow through (IK 4557).
A word to doctors: a doctor is Hashem's messenger in healing, and his own joy and trust in Hashem affect his work (IK 2072, 5465, 6262).