The opioid crisis is being fueled by anonymous online sales on the dark web, where buyers purchase fentanyl and other drugs using special browsers and virtual currencies such as Bitcoin, The New York Times reports.

Law enforcement officials say Internet sales of synthetic opioids such as fentanyl are on the rise. They are frustrated in their attempts to crack down on these sales because of their anonymous nature. Enough fentanyl to get almost 50,000 people high can fit into a standard first-class envelope, the article notes.

A leading dark web site, AlphaBay, last week had more than 21,000 listings for opioids and more than 4,100 for fentanyl and similar drugs. The number of fentanyl listings on AlphaBay and other dark web sites has been steadily increasing.