SEATTLE (AP) _ Researchers at the University of Washington said Thursday they have developed an inexpensive method that can expose cyanide and other poisons in foods and drugs.

Under the method, a compound containing iron would be added to the food or medicine, said David Honigs, assistant professor of chemistry.

If a poison were added, it would react with the compound to produce a dark stain, he said. The chemical change occurs almost as soon as the contaminant is added, Honigs said.

Honigs, 27, said he and two graduate students developed the method after a series of recent fatal tamperings of over-the-counter painkillers.

''Hopefully, this will add another layer of protection,'' in addition to tamper-resistant packaging, he said.

''There isn't any totally safe way to package many foods,'' he said. ''We view what we've done as an important additional step to protect consumers from potentially harmful contaminants.''

The university has applied for a patent on the method, which can be used in dry or liquid form and in a variety of packaging. University officials said they hoped to interest a private company in marketing the compound.

Honigs said the method had been effective on chemicals such as cyanide, strychnine and mercury acetate.

The two common forms of iron in the compound already are approved by the FDA as safe vitamin additives in foods such as baby cereal, bread and some gelatin desserts.

Kelly Fry, spokeswoman at the FDA's Seattle office, said the office was unaware of the new treatment, and would have no comment.