To my mind 'remedy' would imply the same thing as 'cure'. That is, something which has completely fixed the problem (for instance the person would no longer feel sick). A treatment would be a procedure to make the person feel better but they might not necessarily be completely recovered by the end of it. As an example: If I have a bad back a treatment might make it feel better but not perfect. Whereas a remedy would completely solve the problem.

If you are asking whether you can use "remedy" instead of "treatment" in a formal medical setting, I would say no. I have never seen the word remedy in any of the biomedical literature I have read. The words treatment and therapy are used however.

A remedy is something that usually works. It is the standard cure - It is better than a treatment - and not as good as a cure. Cures always work. Remedies often work.