Patients with urgent medical problems between 16:00 and 18:00 GMT are still asked to phone the practice, but those with non-urgent requests after 16:00 should use the online system the following morning.
Those without internet access are advised to complete the online form with help over the phone, or in person at their usual surgery.
Manx Care said the new system would make it “easier for patients to access the right care, at the right time”.
The changes would “reduce pressure on the practice”, the healthcare provider said, while “improving fairness in how appointments are allocated”.
It said: “This allows clinical staff to determine the most appropriate next steps, whether that is an appointment, advice, or onward referral based on the patient’s individual needs.”