When that prescription runs out — and it will — users who are dependent are often drawn to a substance far less expensive and, unfortunately, extremely accessible on the black market: heroin. In every corner of Pennsylvania, heroin is classified as the number-one drug threat to its residents, but Philadelphia has the highest purity for the lowest cost compared with other major cities. Now, with synthetic opioids like fentanyl and carfentanil on the rise, lethal doses of heroin are landing in the hands of our friends and neighbors.