LaQuenta Caldwell-Moody considered it improper when a pharmacy sales representative tried to take her teenage son, when he was still a minor, to dinner without her.

The salesman was the father of someone with hemophilia, the same disease her son has. But this invitation seemed mercenary, taking advantage of their friendship and shared illness to try to woo the business of her son, Austin Caldwell, whose drug treatments cost more than $1 million a year.

“He’s a cash cow,” said Ms. Caldwell-Moody, who lives in Concord, N.C. “He’s wanted by a lot of people.”

Drugs for hemophilia are so expensive and therefore so lucrative for the pharmaceutical industry that they have created an unusual conflict of interest, blurring the lines between being a patient and drug seller. More and more, manufacturers of hemophilia drugs and the specialty pharmacies that dispense the medicines are hiring patients and their relatives to gain an inside track and access in selling their products.