Researchers posing as heroin users seeking help contacted hundreds of treatment clinics in U.S. states with the highest overdose death rates.

The study found the “secret shoppers” were denied appointments much of the time, especially if they said they were insured through Medicaid.

Scientists wanted to understand why more people aren’t treated with an effective anti-addiction drug available in doctor’s offices.

They tried reaching 546 prescribers with working numbers listed on a government website. If callers said they would pay cash, 38% were told no appointments were available. But 46% were denied appointments when they said they were on Medicaid.

Medicaid is the federal and state insurance program for low-income patients. It covers a sizable portion of people with opioid addiction. The study appears Monday in Annals of Internal Medicine.

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