EUGENE — The new state pseudoephedrine registry appears to be making a dent in the illegal drug trade built on turning the over-the-counter cold remedy into methamphetamine, authorities say.

The number of meth labs found in Oregon fell by one-third the month after pseudoephedrine was moved behind the counter while state officials crafted the registry rule last year.

It fell again by nearly half after the registry began in April, according to Oregon State Police figures.

The registry rule requires the state’s 800 pharmacies to record the identity of customers purchasing cold medications containing pseudoephedrine, which can be reformulated to produce illegal meth.

“It’s a huge investigative tool,’’ said state police Detective Robert Edwards.

He used the rule last month to obtain a search warrant to investigate a man serving a jail sentence on a drug possession conviction.

Lonnie Lewis Harrison, 33, is listed by the federal Bureau of Alcohol Tobacco and Firearms as a multistate offender and potential defendant for prosecution under the federal armed career criminal statute, which carries a mandatory 30-year prison term, according to court records.

In a search warrant affidavit, Edwards reported that the registry showed Harrison acquired 34,500 milligrams of pseudoephedrine in purchases at nine separate pharmacies. Harrison bought half the amount in one 11-day period in July, Edwards said.

A search of Harrison’s residence turned up methamphetamine and a half-dozen firearms. A search of storage units rented by Harrison turned up lab equipment and chemicals, including red phosphorus and iodine, used in making meth, according to court records.

Harrison is the first case where the registry was key to a meth lab investigation since the law went into effect, according to state police Lt. Mike Dingeman, who supervises a state police drug task force.

Police busted 447 meth labs in 2004. The total for 2005 fell by 59 percent to 185 labs, Dingeman said. “The drop started immediately after that rule went into effect,’’ he said.

In July, Oregon will become the only state in the nation where pseudoephedrine will become a prescription drug, forcing meth cooks to go out of state to shop for pseudoephedrine, Dingeman said.

If they stay out of state, the registry will have accomplished another of its goals: lowering the Oregon public’s cost for cleaning up labs so that those resources can be devoted to enforcement and treatment, Oregon Board of Pharmacy Executive Director Gary Schnabel said.

Information from: The Register-Guard, http://www.registerguard.com