Marketing and advertising come under the same ruling as the product that is being marketed. If it is something that it is permissible to use and consume, there is nothing wrong with marketing it, even if the manufacturers are non-Muslims. But if the product is haraam in and of itself, then it is not permissible to sell it, advertise it or market it. The evidence for that is the general meaning of the words of Allaah (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment”
And it was narrated from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “When Allaah, may He be glorified and exalted, forbids a thing, He also forbids its price.”
Narrated by Ibn Hibbaan (11/312); classed as saheeh by al-Albaani in Ghaayat al-Maraam (318).
Ibn Rajab said in Jaami’ al-‘Uloom wa’l-Hukam (415-416):
This is general and comprehensive, and applies to everything the use of which is haraam. End quote.
The scholars are unanimously agreed that whatever is permissible to use and consume, it is also permissible to buy and sell it, and marketing is no more than selling the product and advertising it, so it comes under the same ruling.
As your marketing is for medicines that are beneficial in the treatment of various diseases, there is no sin on you for that in sha Allaah, and it does not matter whether the owners of the company are Muslims or not, because what matters is the product that you are marketing, not who the owners of the company are. But two important points should be noted here:
1 – Overstepping the limits set by Allaah in marketing and advertising – as many marketers do – by describing the product as other than what it actually is, is a kind of lying and bearing false witness.
2 – You should not do what many marketers do, which is giving gifts to doctors and managers of clinics so that they will agree to deal with them and buy their products, because that open the door to paying the doctor for prescribing this medicine to his patients even if there is some other medicine that is better, and this is a betrayal of trust.
It says in Fataawa al-Lajnah al-Daa’imah (23/572):
With regard to working in advertising companies and company representatives, some – or most – companies have now started to work by giving gifts or bribes, and the rep is afraid of losing his position in the company if he does not do that. Most doctors will never prescribe a specific medicine for a patient if there is no gift from the company, so he prescribes the medicine sold by the one who brings him gifts. So he finds himself forced to get involved in this gift-giving. What is the ruling on that? Is it a case of necessity because he is afraid for his job, or not? Especially since he may have financial obligations that he must pay.
The answer was:
The company rep who gives gifts to doctors in order to distribute the medicines of that company instead of others is regarded as giving a bribe; he is regarded as the middleman between the giver and taker of the bribe. The Prophet (peace and blessings of Allaah be upon him) cursed all three, and said: “May Allaah curse the giver of a bribe, the taker of a bribe, and the middleman.” Musnad Ahmad (5/297). End quote.
And Allaah knows best.