A doctor is supposed to be honest, and part of honesty is to prescribe the best and cheapest medicine available to patients. If a doctor prescribes a certain kind of medicine knowing that there is another one that is cheaper and more beneficial, then he is not honest.
It is not permissible for the owners of pharmaceutical companies to attempt to give doubtful offers in a bid to achieve more financial profits. If they did so, they would incur the sin implied in this action as well as the sins of the doctors involved and the salesmen working for them.
To be on the safe side, salespersons can simply and honestly describe the effects of the medicine they are marketing without giving doubtful offers, and then leave the doctors to decide for themselves what to prescribe for their patients. In turn, doctors should prescribe the cheapest and most beneficial medicine to their patients without being affected by the incentives they receive from pharmaceutical companies.
Dr. Mahmoud `Akaam, Professor of Shari`ah at Halab University, Syria states the following: It is a breach of a doctor’s duties that he prescribes medicine produced by a certain pharmaceutical company on the basis of the offers he receives from that company, regardless of how beneficial this medicine may be to patients.

This is a form of cheating, which is defined in the Shari`ah as deceiving someone by presenting him/her something whose specifications are different from the specifications he/she needs. The Prophet (peace and blessings be upon him) is reported to have said, “He who cheats us is not one of us [is not a faithful Muslim].”
A doctor’s primary role is to diagnose the patient’s condition and then to honestly prescribe the medicine he considers to be effective in treating the disease.

Moreover, Dr. Ahmad Sa`eed Hawa, professor of Fiqh and its principles at the Jordanian Universities, states: The principal duty of a doctor is to prescribe to his patients the medicine that he considers to be most beneficial in treating their cases. He is not to be affected by the incentives which pharmaceutical companies offer him in order to prescribe the medicine they produce.

If a doctor prescribes a certain medicine because he receives incentives from the company producing that medicine, while knowing that there is another kind that is cheaper and more beneficial, then he is violating the dictates of honesty. He and the company that deals with him share in the sin implied in such an act.
A doctor may accept a present from a pharmaceutical company when there is an actual bond [friendship, for example] between the doctor and the owner of the company. In such a case there should be no pressure on the doctor to prescribe the medicine produced by the company to treat his patients.

Dr. Hamad ibn Ibrahim Al-Haidari, a professor of Shari`ah at the Islamic University of Imam Muhammad ibn Saud, adds:

What is called a “present” given by a certain pharmaceutical company to a doctor so that he prescribes its medicinal products is no more than a bribe. This is because it incites the doctor to prescribe medicine to his patients, not on the basis of the cheapest price, (which is more beneficial to them) but on the basis of the benefits he will obtain from the company. If it wasn’t for such presents, the doctor may have prescribed the medicine needed by the patient at an affordable cost.
The Prophet (peace and blessings be upon him) said: “Religion is to be faithful… to Almighty Allah, His Book, His Messenger, the rulers of the Muslims, and to common Muslims.” (Reported by Muslim)
The actions of many pharmaceutical companies as raised in the question, are contradictory to the faithfulness Muslims are required to have towards one another according to the above mentioned hadith. When a certain pharmaceutical company gives incentives to doctors to prescribe its products to patients, it seeks to achieve marketing benefits at the expense of the patients who could otherwise use other cheaper medicines having the same effect.
Likewise, exaggerating in describing the benefits and effects of products in any way is also a kind of cheating that is unlawful in Islam. The Prophet (peace and blessings be upon him) said, “He who cheats us is not one of us [is not a faithful Muslim].”
To be on the safe side, marketing companies and their representatives are to describe their products without exaggeration. If they fail to do so, they may be guilty of committing doubtful things. So they should beware.

Finally, Hamed Al-`Attar, a researcher in Shari`ah at www.islamOnline.net adds:

When a patient goes to a doctor he has complete trust in him. He is certain that the doctor will do all that is in his patient’s best interest. It never occurs to the patient that the doctor may betray such a trust.
When the doctor prescribes a certain kind of medicine while he knows that there is another medicine having a similar effect yet is more beneficial for the patient than the one he prescribed, he will be committing a number of mistakes that render him sinful.
First, by doing so he is violating an Islamic duty upon him towards his fellow Muslims as derived from the Prophet’s words: “Religion is to be faithful … to Almighty Allah, His Book, His Messenger, the rulers of the Muslims, and common Muslims.”
Second, he is betraying his patient’s trust in him.
Third, the presents or the incentives that he has taken from the pharmaceutical company are regarded as a bribe, which in turn may lead him to enter the Fire in the Hereafter.
Fourth, he is violating one of the requirements of his humanitarian mission, that is, honesty.
It is important to note that if a doctor accepts the presents given by pharmaceutical companies, it would lead them to try to tempt more doctors instead of seeking to improve the quality of their products. In the end, the one who pays the price is the patient, who can be said to have fallen prey to the greed of both pharmaceutical companies and doctors.
To be on the safe side, doctors should not take presents from pharmaceutical companies. They should, rather, prescribe medicine that is in the best interest of their patients in terms of their health and income.