MINNEAPOLIS (AP) — Nurse practitioners are becoming vital to health care in Minnesota, which is seeing the demands of aging patients outpace the supply of doctors.

The number of licensed nurse practitioners has grown from almost 3,900 in 2014 to more than 5,600 this year. The growth comes after state lawmakers enacted licensing standards for advanced practice registered nurses.

The growing popularity of nurse practitioners is raising safety concerns because doctors complete up to 10 times more hours of clinical practice during their education than nurse practitioners.

The Montana-based Center for Interdisciplinary Health Workforce Studies published a study last week about the differences between doctors and nurse practitioners. Center Director Peter Buerhaus says medical groups should employ both doctors and nurse practitioners to complement each other.