Illicit drugs increasingly are being traded in hidden online marketplaces known as “crypomarkets,” The Wall Street Journal reports.

Revenue for illicit drugs in these online markets has doubled since 2013, while transactions have tripled, according to a study released by RAND Europe, a policy research institute.

Crypomarkets are predominantly run by drug sellers in the United States, Australia and the United Kingdom. Monthly revenue is estimated at between $12 million and $21.1 million. Most of the revenue is being generated by customers purchasing drugs in large quantities, suggesting they intend to resell them. Marijuana sales represent 37 percent of total revenue, followed by cocaine and amphetamines.

These online markets have emerged since the Federal Bureau of Investigation closed the illicit online marketplace Silk Road in 2013.