It's illegal for an employer to harass their employees about any medication they take as long as the medication they take does not interfere with their job. If they do they are in violation of the ADA and other various  labor laws meant to protect private health information. Just because they may pay a part of their employees health insurance (or even all of it) does not give them the authority to dictate how that insurance is used.
https://www.healthit.gov/providers-professionals/patient-consent-electronic-health-information-exchange/health-information-privacy-law-policy