More than three-quarters of Web sites that offer highly addictive medications do not require a prescription, according to a study released Wednesday.

Powerful pain killers like morphine and oxycodone, and stimulants such as amphetamine, are among the so-called controlled substances regulated by the Drug Enforcement Agency because of their high potential for abuse and addiction. But findings by Columbia University researchers highlight how easily those narcotics and stimulants can be purchased online.

"Anyone of any age can obtain dangerous and addictive prescription drugs with the click of a mouse," said Joseph Califano, who heads Columbia's National Center on Addiction and Substance Abuse, which issued its fifth annual report on online drug dispensing.

Of the 365 sites selling controlled substances online, Columbia researchers found 85 percent did not require a doctor's prescription.

The number of online establishments selling the drugs fell 37 percent from last year, but researchers said the decline was likely due to law enforcement efforts to crack down on online drug trafficking.

About 80 percent of all online prescriptions are for controlled substances, compared with just 11 percent of prescriptions filled at traditional pharmacies, according to the DEA.

The Senate earlier this year passed a bill that would explicitly ban the delivery of highly regulated drugs over the Internet without a prescription. The bill is pending in the House.

As politicians consider stricter standards for online pharmacies, government regulators are actually easing restrictions on how doctors can prescribe controlled substances.

The DEA last month proposed allowing doctors to prescribe such drugs with online prescribing software. The programs, which replace handwritten notes, allow physicians to send prescriptions directly to a pharmacy from their office.

Insurers like Humana Inc. and large employers like Wal-Mart Stores Inc. have lobbied to allow electronic prescribing of controlled drugs. They argue that online prescriptions are an easy way to avoid deadly medication errors and reduce health care costs.

Many doctors have resisted the technology because of the initial investments required to buy and install it.

By paving the way for electronic prescribing of widely used controlled substances, experts expect more doctors will embrace the technology.