It is not permissible for a doctor to accept gifts from pharmaceutical companies

We see intense competition among the different pharmaceutical companies whose representatives come and distribute gifts to doctors, such as pens on which the name of the medicine is written, or watches or recorders, etc, in return for the doctor prescribing this medication for patients. It should also be noted that there is a department in the company that is dedicated solely to advertising. 
Sometimes the company will give the doctor a specific gift in return for prescribing a specific amount of their drug, and sometimes they will give him a gift in return for his prescribing it without specifying any particular amount. And sometimes drugs are equally effective but they are produced by a number of companies with different brand names, so the doctor prescribes the brand of the representative who has visited him most recently and brought him a gift. 
Is advertising in this manner halaal or haraam, and what is the ruling on giving these gifts to doctors?.