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. For a halachic question, consult a competent rav.
The Rebbe's guidance where medicine and halachah meet:
Two kinds of expert, each in his own field. A medical decision with halachic implications needs both an expert doctor for the medical side and a qualified rav for the halachic side, and each question should be brought to the right one (IK 4935, 5634, 1411). Before a procedure, arrange with the doctor that it is carried out according to halachah; where possible, avoid scheduling close to Shabbos or during the Nine Days (IK 10190). Delaying a procedure can sometimes be considered, but the final medical authority is the doctor (IK 7748).
Health comes first. Pikuach nefesh on Shabbos (IK 176). When health requires it, follow the medical need even where it affects a custom, such as eating before davening (IK 7070). Where older sources or Chassidic tradition caution against a procedure, such as tooth extraction, follow current expert medical advice (IK 2572, 4349). When an expert deems a procedure necessary, follow through as the rav directs (IK 4557). Stringency against medical instructions is discouraged (IK 4725). One may consult a non-Jewish doctor (IK 3512).
Caution with irreversible steps. Irreversible procedures should be considered only when truly necessary and after consulting several experts and exhausting the alternatives (IK 6429, 7152). Weigh both the benefit and the risk of a treatment (IK 5547).
Pregnancy, fertility and family life. Questions in these areas are highly individual and were answered case by case; the Rebbe's consistent instruction is to consult both an expert doctor and a qualified rav (IK 828, 1122, 1232, 1411, 1697, 2842, 2876, 5408, 5634, 5715, 7152).
Communal matters. The Rebbe also wrote about the halachic issues in autopsies (IK 9275), bris milah practice (IK 5410), and the halachic questions surrounding adoption, urging greater awareness among both rabbis and laypeople (IK 8924, 8954).