Cris Wibby joined online mental-health company Cerebral Inc. as a nurse practitioner last spring, looking for the flexibility to work from home while helping new patients.

She began treating adults with attention deficit hyperactivity disorder and offering prescriptions for powerful stimulants such as Adderall. She could do so because the federal government relaxed a rule requiring an in-person visit before such substances can be provided, and nurse practitioners are allowed to prescribe controlled substances in many states. Ms. Wibby said patients clamored for the medication, and she said she sometimes struggled to diagnose ADHD during 30-minute video calls that she said sometimes felt could be too brief.