The drug, used to treat hyperactivity, is taken by about 900,000 patients, mostly children.

PHOENIX, Ariz. -- A nationwide shortage of the prescription drug Ritalin has left some pharmacey shelves bare of the medication, which many parents say is critical to keeping their children under control.

Strict federal controls on supplies have parents accross the country scrambling to find the drug, used mainly to treat children with extreme hyperactivity and attention-deficit disorder, a learning disability.

Jacque Redfield of Phoenix said her 11-year-old son, Kenneth, was without his daily dose of the medication for two weeks and nearly accidently hanged himself.

"He was bouncing off the walls," Redfield, 33, said. "I told him to go outside and play and work off some of that energy. He hit a tetherball so hard the cord wrapped around his neck and he was hanging there in the back yard.

"I hate to think what would have happened if I hadn't been watching him."

She managed to get a three-week prescription filled on Tuesday, and she said Kenneth is back on the medication.

"When this runs out, I'm not sure what I'll do," she said.

Several chain pharmacies reported shortages of the drug, especially in the popular 10-milligram tablet form.

Ritalin is a central-nervous stimulant that helps bring hyperactivity under control.

About 900,000 patients nationwide, mainly children, take Ritalin or the generic drug methylphenidate.

It is an amphetimine, and its supply is controlled by the federal government. Ritalin is in short supply, the manufacturer says, because the federal Drug Enforcement Administration, which sets quotas on how much of the drug can be produced, set the amount too low.

"There is a national shortage," said Todd Forte, a spokesman for Cib-Geigy Corp. of Summit, N.J., which manufactures Ritalin. "We're getting calls from all over.

DEA officials in Washington revised the quota Oct. 7, allowing 1,600 additional kilograms of the drug to be produced, bringing the total to 5,000 kilograms for the year. That's up from a total of 1,768 kilograms in 1990, according to the DEA.

Ciba-Geigy is working "24 hours a day" to manufacture more of the drug, but it likely will be next month before new shipments reach pharmacy shelves, Forte said.

Joyce McDonald, a spokewoman for the DEA in Washington, said there was a delay in revising the quota, usually done at midyear, because the Justice Department wanted to take a closer look at the situation.

"There is a great potential for abuse with this drug," she said. "We don't want an overabundance of this drug on the market."

"All of this has put us in a very awkward position," said Dr. Habes Sawalqah, a Phoenix pediatrician. "This isn't the kind of medication you can take a child off and put on something else."

Deborah MacLeod, 33, of Scottsdale, said her 10-year-old son, Alexander, was without medication for only a few days, until she searched dozens of pharmacies and found a short supply of the drug.

Without it, she said, her son quickly goes from being a well-behaved honor-roll student to a child who can't cope int the classroom.

"It's like a Jekyll and Hyde situation," she said.