Drug companies have long courted doctors with gifts, from speaking and consulting fees to educational materials to food and drink. But while most doctors do not believe these gifts influence their decisions about which drugs to prescribe, a new study found the gifts actually can make a difference – something patient advocates have voiced concern about in the past.

New evidence, published Thursday in the journal The BMJ, shows that doctors who receive gifts from drug companies are more likely to prescribe or recommend their products, at least when it comes to brand-name medicines used to prevent blood clots or treat diabetes. Other studies have also suggested an association.

Under President Barack Obama's health care reform law, commonly known as Obamacare, drug companies are now required to publicly report their payments to doctors, allowing for more specific analysis. Researchers for the latest study used the tool created created as a result of the law, called Open Payments, which is run by Centers for Medicare and Medicaid Services.

The findings suggested that for oral anticoagulants, or blood thinners, and non-insulin drugs used to treat diabetes, payments to specialists were associated with greater prescribing of brand-name drugs than payments to non-specialists, who receive fewer gifts.

The study found that when a drug company spends $13 on a doctor, then they will later see 94 additional days of prescriptions for brand-name anticoagulants and additional 107 days of prescribing brand-name drugs to treat diabetes. These are referring to the actual days-supply of medication a patient picks up at the pharmacy.

For instance, explains Dr. Will Fleischman, lead author of the study and clinical assistant professor in the department of emergency medicine at the University of Maryland School of Medicine, a regular habit is for a pharmaceutical sales representative to take a doctor out to lunch. Later, that doctor may doctor prescribe a medication to a patient for about a three-month period.

"We're not saying that the payment caused the prescription but that it's tied to the prescription," Fleischman says. "For each payment in a region, they could expect three months of a medication being prescribed over a generic medication.

Payments for food and beverages or for educational materials appeared to have less of an impact than other types of gifts like speaking engagements, at least in the case of diabetes drugs. Gifts like travel, speaking and consulting fees would result in 375 more days of filled prescriptions than if the gift was in the form of educational materials or meals.

"This influence on prescribing can potentially negatively affect patients through inappropriate prescribing, or more likely prescribing of more expensive branded drugs when cheaper, generic alternatives exist," authors of the study write.

They examined nearly 46 million prescriptions to about 10.5 million recipients of Medicare, the government's coverage program for adults 65 and older. The prescriptions were written by nearly 624,000 doctors for the last five months in 2013 and the full year 2014. Specialists in the study were defined as cardiologists and hematologists for the anticoagulants, and endocrinologists for the diabetes drugs.

Authors of the study caution that their findings do not necessarily suggest that payments by pharmaceutical manufacturers are harmful for patient care.

"Patients may benefit from physicians being made aware of newly approved, effective treatments that may have fewer adverse effects, reduce the need for monitoring tests, or improve adherence," they wrote.