The National Center on Addiction and Substance Abuse says more Web sites are popping up, offering prescription medicines like OxyContin, Vicodin, Xanax and Ritalin.
Center chairman Joseph Califano says the government needs to tighten laws that regulate Internet sales of controlled drugs.
He says the online market has become a "candy store" for drugs.
The center has checked Web sites for one week every year over the past three years.
The study being released today has more sites offering drugs without prescriptions.
The number has been increasing since the annual review began three years ago.
Califano says nine out of ten sites selling controlled drugs over the Internet don't require a prescription.
The Drug Enforcement Agency is making arrests.
DEA agent Steve Robertson says a Dallas-Fort Worth operation was recently shut down because it was based on a phony doctor.
But Robertson says there are legitimate businesses on line that obey the rules.