If it is anything immoral then it is haram. In majority of cases our conscience tells us what is moral and what is not.

There can’t be a pre-determined answer to your question since there can be many variables involved. For example, if there are two pharmaceutical companies offering materially the same product, the doctor has to pick one anyway. I don’t see anything immoral if the doctor prefers one brand over another in exchange for a commission. On the other hand, if a doctor knows that one product is inferior than the other but still prescribes it because he gets more commission, then this is certainly immoral.

In recent Q&A Session with Ghamidi Sahab on Questions selected from ASK GHAMIDI Platform:

For answer to your question, please refer to the video below from 1:10:05 to 1:11:51

https://youtu.be/6IFA34jO72I?t=4205