Federal health officials on Aug. 28 instructed 21 websites to stop selling "potentially dangerous, unapproved and misbranded" opioids online, according to CNBC.
CoinRX, MedInc.biz, PharmacyAffiliates.org and PharmaMedics operated the 21 websites.
So far this summer, the FDA has targeted 70 websites selling opioids. Some drugs, the FDA found, were a form of pressed fentanyl sold as opioids.
"As we see doctors prescribe fewer opioids, we're fearful that more and more of the new addiction is going to shift to illicit sources, and a lot of those illicit sales are taking place online," Scott Gottlieb, MD, FDA commissioner, told CNBC's "Squawk Box".
On June 27, the FDA convened technology companies like Facebook and Google to discuss ways to curb online opioids sales on their sites.