If someone offers you a red joint, don't smoke it.

The red color will probably mean that the marijuana has been treated with the dangerous herbicide, paraquat.

Mexico has been spraying paraquat on illegal marijuana fields for the past three years as part of a U.S. aided program to reduce the amount of marijuana smuggled into the United States.

The spray, however, had two unfortunate side effects.

The first was that it turned the marijuana a yellowish-gold color, leading uninformed users to mistake it for high-quality Mexican marijuana known as Acapulco Gold.

Then in March, the U.S. Department of Health, Education and Welfare warned of a more serious problem. It said smoking marijuana that had been sprayed with paraquat could result in irreversible lung damage.

Mexican authorities insist that the health risks posed by paraquat are nowhere near as serious as suggested in the United States.

Nevertheless, they have now decided to mix a red dye into the paraquat in an effort to alert prospective marijuana users. Mexican authorities began experimenting with the dyes two weeks ago in the fertile plantations of the Sierra Mardes.

"The leaves come out full of red spots, like they have been splashed with red ink," said Deputy Attorney General Fernando Baeza. "We are still hoping to get the whole plant to turn bright red, but marijuana green is really very intense. I think we can perfect the method, though."

"Even if the joint does nto look bright red to start with, it will under ultraviolet discotheque lights," said Baeza. "And if you smoke enough of it, your lips will look the same."