A method is provided for authenticating, and establishing and tracking eligibility of a patient to purchase an over-the-counter (OTC) drug from a retailer, the patient using a client device or terminal and the retailer having a server. The server is configured to redirect the client device or terminal to a backend platform to authenticate the patient, access a storage to search for any information that indicates the patient is ineligible to purchase the OTC drug; and without any information that indicates the patient is ineligible, administer a questionnaire to the patient to assess the eligibility for the OTC drug. And the backend platform redirects the client device or terminal back to the server to complete the online sale of the OTC drug to the patient when the patient is eligible, or cancel the online sale of the OTC drug to the patient when the patient is ineligible.