Ten Connecticut prescribers, including a Derby nurse practitioner who is under investigation by the state, were responsible for more than 22 percent of the state’s Medicare spending on potent narcotics in 2013, new federal data show. More than 4,300 Connecticut clinicians, mostly physicians, wrote Medicare prescriptions for oxycodone, morphine and other Schedule II drugs, which have a high potential for abuse and addiction, at a total cost of $40 million. But a handful of those providers accounted for the largest share of those prescriptions, an analysis of the data show. Topping the list, in both the number of prescriptions and cost, was Heather Alfonso, an advanced practice registered nurse (APRN) at the Comprehensive Pain & Headache Treatment Centers, LLC, in Derby, who wrote out 8,523 prescriptions, at a cost of $2.7 million. Statewide, the average number of prescriptions per clinician was 104, and no other prescriber had more than 5,000 Schedule II claims. The average cost per prescriber was $9,138; the next closest individual cost was $1.8 million.