BOSTON — When Dr. Michelle Johnson scribbles out prescriptions, the next stop for many of her patients is the gym, not the pharmacy.

Doctors treating chronic health problems increasingly are prescribing exercise for their patients — and encouraging them to think of physical activity as their new medication.

In one such program run by a health center in Boston’s Roxbury neighborhood, primary care physicians, internists and psychologists prescribe access to a gym for $10 a month.