Drugmaker Merck & Co. has narrowed its research focus in the hope of raising profit, but type 2 diabetes remains part of the program because the market is huge and will likely grow as Americans and people in other countries get older and heavier.

Merck said Friday that it would create a "global registry" of about 20,000 patients to evaluate their real-world experience with medication, blood sugar levels, diet, exercise, use of health care, and quality of life. Merck will collaborate with academic researchers at more than 900 sites in the United States, Germany, France, and Japan.

Clinical trials to test efficacy and safety are done to gain approval of medicines, but less research is done after the sale. Peter Stein, vice president of diabetes and endocrinology at Merck Research Laboratories, said 40 percent to 60 percent of patients did not take medicine they had obtained to treat chronic diseases like diabetes. Merck is working on a weekly medication for those who cannot seem to remember to take a daily pill.

"There remains a huge medical need, with half of the patients not at their A1C goal," Stein said, referring to a test to measure blood sugar levels during the previous three months. "Merck has a deep and broad interest in the management of patients with diabetes."

The U.S. Centers for Disease Control and Prevention said an estimated 29.1 million Americans have diabetes, with 8.1 million of them undiagnosed. The International Diabetes Foundation estimated 382 million diabetics. . The vast majority of cases are type 2, in which the pancreas produces insufficient insulin and which can be treated with diet, exercise, and oral medication.

Merck is among the drugmakers trying to sell that medication. Januvia is Merck's best-selling drug, with $4 billion in sales last year. Janumet is a slightly different version, and the combined 2013 sales were $5.83 billion. But the pace of revenue growth slowed in the last two years, with 2014 first-quarter combined sales at $1.3 billion.