A large majority of 365 Internet sites that advertise or sell controlled medications by mail are offering to supply the drugs without a proper prescription, according to a new study. The online trade is stoking the rising abuse of addictive and dangerous prescription drugs, the authors and federal officials say.

Drugs offered online include generic versions of opiates like OxyContin, methadone and Vicodin, which are legitimately prescribed as painkillers; benzodiazepines like Xanax and Valium, which are prescribed for anxiety; and stimulants like Ritalin.

Federal and state efforts to crack down on Internet sales appear to have reduced the number of sites offering such drugs, from 581 last year, said Joseph A. Califano Jr., director of the National Center on Addiction and Substance Abuse at Columbia University.

"Nevertheless, anyone of any age can obtain dangerous and addictive prescription drugs with the click of a mouse," Califano said. The center is issuing the study, the latest of five annual surveys, on Wednesday.

The Drug Enforcement Administration found that 85 percent of all Internet prescription sales involved controlled drugs, compared with just 11 percent of those filled through regular pharmacies, suggesting that online sales often are destined for misuse.

"Abuse of prescription drugs has exploded among college students, and we think that one way they get these drugs is over the Internet," Califano said. The use of prescription opioids and anxiety drugs, especially in combination, accounts for a growing share of deadly overdoses nationwide.

"The Internet made it easy for the drug dealers to sneak into your living room," said Francine Haight of La Mesa, Calif., whose son Ryan died in 2001 at the age of 18 from an overdose of hydrocodone, generic Vicodin, which he had secretly ordered online with a debit card. An A-student and varsity tennis player, he had claimed in an online questionnaire to be a 25-year-old with back pain, got his prescription and was mailed the drug. Haight, a registered nurse, has since fought against online sales.

Federal law bars dispensing dangerous medications without a prescription from a doctor who has a bona fide relationship with the patient. But officials have had a hard time catching up to rogue Internet pharmacies that sometimes ship the drugs from foreign countries in disguised packages.

For the last several years, the Drug Enforcement Administration and others have worked to halt the illegal trade and prosecute involved doctors and suppliers, with limited success.