Health and medicine is very far from deterministic. Your drug interaction checker is deterministic because the non-determinism is handled at a higher level (the doctor / patient interaction) in the health care system. Individual patients often respond wildly differently to the same medicine even in the absence of drug interactions.

Similarly the non-determinism in ChatGPT should be handled at a higher level. It can suggest possibilities for diagnosis and treatment, but you should still evaluate those possibilities with the help of a trained physician.