Praise be to Allah.
It is not permissible to do such things, and it is regarded as a bribe which is haraam, because it makes the employee limit himself to the company that has given him a gift and ignore the other companies. This is consuming people’s wealth unlawfully and harming others. What you must do is to avoid this and warn others against it, because the Prophet (peace and blessings of Allaah be upon him) cursed the one who gives a bribe and the one who takes it. We ask Allaah to keep us safe and sound.
From Fataawa al-Lajnah al-Daa’imah, 23/576
There is another wrongful action which the questioner refers to, which is:
This action may also harm the patient, as the doctor may prescribe this medicine to him, in the hope of getting a gift from the company that produces it, even though some other medicine may be more beneficial to the patient.
And Allaah knows best.