Companies pay doctors millions of dollars to promote not their most innovative or effective drugs, but some of their most unremarkable.

In the last five months of 2013, drug makers spent almost $20 million trying to convince physicians and teaching hospitals to give their freshly-patented drugs to patients, but many of them are near-copies of existing drugs that treat the same conditions.

A hefty portion are also available as generics, chemically identical copies that work just as well at a fraction of the price. And still others have serious side effects that only became apparent after they were approved by the FDA.

That's all according to a thorough analysis from ProPublica's Charles Ornstein and Ryann Grochowski Jones, who combed through federal data on payments made by pharmaceutical and medical device companies to doctors and teaching hospitals and released publicly for the first time last fall under the Affordable Care Act. That data is now available (in a semi-understandable form) on the government website Open Payments.

So why are companies paying so much to try to get doctors to prescribe their products?

In a word: competition.

In ProPublica's list of the top 20 drugs that companies are paying the most to promote, nine had a competitor also ranked within the top 20 that treated the same condition. Most of these competing drugs treat diabetes, schizophrenia, blood clotting, and chronic obstructive pulmonary disease.

Here are the top 20 and the conditions they're used to treat:

1. Victoza, type-2 diabetes 2. Eliquis, anti-clotting drug 3. Brilinta, blood-thinner 4. Invokana, type-2 diabetes 5. Latuda, schizophrenia 6. Xarelto, anti-clotting drug 7. Humira, arthritis, ankylosing spondylitis, Crohn's disease 8. Tudorza, chronic obstructive pulmonary disease 9. Daliresp,chronic obstructive pulmonary disease 10. Abilify Maintena, schizophrenia 11. Abilify, schizophrenia 12. Linzess, irritable bowel syndrome with constipation, chronic idiopathic constipation 13. Pradaxa, anti-clotting 14. Tradjenta, type-2 diabetes 15. Belviq, weight loss drug 16. Copaxone, multiple sclerosis 17. Samsca, hyponatremia (low sodium levels in the blood) 18. H. P. Acthar, infantile spasms, multiple sclerosis, endocrine disorders, arthritis, lupus 19. Symbicort, asthma 20. Aubagio, multiple sclerosis

Take multinational pharmaceutical company AstraZeneca's blood-thinning drug Brilinta, for example, ranked third in ProPublica's list of the highest payments to doctors. One of Brilinta's biggest competitors, Plavix, has been available generically since 2012 at a fraction of the price.

In order to make a profit in such a crowded market, producers of new drugs — who have often spent a fortune on research and development — must make them appealing to the doctors who prescribe them. A study published by Ornstein and Jones in March found that doctors often accept thousands of dollars in speaking and consulting payments from drug companies that also sponsor their research.

During the last five months of 2013 (the period that Ornstein and Jones looked at for their most recent report), Brilinta's manufacturers made 63 payments to doctors, totalling $282,000 in consulting fees. Novo Nordisk, the makers of the most aggressively promoted drug on their list, Type-2 diabetes drug Victoza, spent $816,000 on consulting (doled out in 166 payments).

AstraZeneca told us via email that their efforts are geared towards educating the healthcare community and bringing new medicines to patients.

Novo Nordisk said in an email that the company "work[s] closely with physicians to advance the understanding of the appropriate use of our medicines. Because Victoza falls into a relatively new class of drugs, additional education is required to educate physicians."

Here's the definition of "consulting" from the US Centers for Medicaid and Medicare:

Payments made to physicians for advice and expertise on a particular medical product or treatment, typically provided under a written agreement and in response to a particular business need. These payments often vary depending on the experience of the physician being consulted.

It's super vague, which is part of the reason advocacy organizations have been pushing drug makers to be more transparent about their payments in recent years. The drugs we're getting, the thinking goes, should be the best that are out there rather than the ones payments from drug companies may have convinced doctors to prescribe. (Researchers recently found that such payments, while they may not technically compel doctors to do anything in particular, do indeed affect which drugs doctors prescribe to their patients.)

At least partially as a result of efforts by advocacy groups to push drug makers to be more transparent about the money they give to doctors, consulting payments have begun to drop a bit.

Hopefully, the trend will continue. As of this fall, as part of the Affordable Care Act, all drug makers were required — for the first time in history — to publicly disclose their payments to doctors.

Look up your own doctor here, and read ProPublica's full investigation here.