A lethal drug used to put animals to death, which is also fatal to humans in small doses, was among a cache of medications stolen late Tuesday from a veterinary clinic in Woodbridge, according to Prince William County police.

The drugs, valued at $750, were stolen between 8:30 p.m. and 9:30 p.m. from the Woodbridge Animal Hospital at 13312 Jefferson Davis Highway, said police spokeswoman Kim D. Chinn.

The substances included 1,000 milliliters of sodium pentobarbital, known as Succumb, which is used to put animals to death at a dose of one milliliter for each 10 pounds of animal. The drug is fatal to humans at a dosage of 10 to 15 milliliters, said James McDonald, owner and director of the hospital.

"That's my biggest fear, that someone took this stuff and doesn't realize what they have got, the lethality of the drug," McDonald said. "I'm concerned for what could happen if this drug gets in the wrong hands out on the street."

Police reported the theft yesterday to the Drug Enforcement Administration. Sale of the drugs is prohibited except by licensed dealers to licensed medical professionals.

The burglar apparently entered the clinic by jimmying open a window in an isolated room where drugs are stored in a double-padlocked cabinet, police said. Along with the Succumb, the thief took $650 of an anesthetic liquid inhalant used to put pets to sleep during surgery; a depressant drug in pill and injectable forms was also stolen.

Police found footprints of a shoe they believe may have belonged to the burglar. McDonald said some of his 12 employees were questioned, but he does not believe the burglary was committed by anyone associated with his office.

But the method of entry and the drugs taken indicate the burglar may have been familiar with his office, McDonald said. The thief broke into the only window not equipped with heavy mesh screening. The theft occurred between the time the office closed and the arrival of the night staff. Cash and expensive equipment were not stolen and hospitalized animals were not harmed.

Police said they believe the burglar may be a drug user who may take the drugs to get high.

"Whoever took this better not use it," McDonald said. "This whole thing has been very confusing to me because they didn't do much damage. They left expensive equipment and didn't touch the cash register, so it doesn't appear to have been financial. This looks like someone who was looking for drugs. And these drugs are very potent stuff."