Does your employer have the right to suggest that you are manic depressive and should take drugs?
Your employer most certainly has the right to make a suggestion.
Of course, you have the right to respectfully decline. If your
employer insists you take drugs for a mental disorder "diagnosed"
by this employer, you have the right to respectfully inform this
employer that it is illegal to practice and prescribe medicine
without a license to do so. Unless, of course, your employer is a
licensed practitioner of mental disorders, but even then, you have
the right to respectfully decline.