بسم الله الرحمن الرحيم
It is not absolutely haram to visit the doctors and introduce the medicines of the related company and offer to prescribe the medicine of that particular company and offer some gifts to the doctors. It is allowable for the doctors to accept the gifts without demand. Yes, if the doctor demands for gift for prescribing the medicines of related company then in such a case it is not lawful to give him gift merely for writing the medicine of that company. Hence in the above mentioned case, your job is lawful.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.