My provincial health card expired mid-treatment. What happens to my ADHD prescription?

An expired provincial health card mid-treatment typically affects billing and coverage, not the idea of the prescription as a document. Pharmacies and physicians usually need valid provincial coverage to bill the public plan. An expired card can mean you pay privately, the fill is delayed until renewal, or the visit cannot be billed as an insured service until the card is current. References: https://www.ontario.ca/page/renew-health-card https://www2.gov.bc.ca/gov/content/health/health-drug-coverage/msp