Although the use of spice has decreased in recent years, according to some surveys, the drug continues to be sold by street dealers, as well as openly on the Internet and at smoke shops and other retailers, as potpourri or incense with brand names like Scooby Snax and Black Diamond.

Spice’s health effects have been underscored in recent months by a surge in emergency room visits and calls to poison centers, for symptoms that can include extreme anxiety, violent behavior and delusions. Intermittent reports from several states suggest that at least 1,000 Americans have died since 2009 after smoking spice. However, the Centers for Disease Control and Prevention does not track national data for spice or other synthetic drugs, including those popularly known as bath salts or flakka.

“There’s a constant influx of these new designer drugs, and toxicology tests can’t keep up,” said Ron Flegel, a forensic toxicologist at the Substance Abuse and Mental Health Services Administration. “You can tell someone’s level of oxycodone intake, but often not the synthetics.”

Once created in Chinese labs, spice chemicals are shipped in powdered form, in packages labeled fertilizer or industrial solvents, through commercial couriers to wholesalers in the United States. Those wholesalers liquefy the powder in acetone or alcohol, apply the liquid to a smokable plant material and package the mix in metallic pouches.

Spice producers have been known to mix the concoction in animal feed troughs, hand-cranked cement mixers and on backyard tarps. The packaged product, often coyly labeled “Not for human consumption,” undergoes no safety testing, and has been found to be contaminated with other chemicals, mold or fungus.

The ease with which synthetic-drug manufacturers modify their chemicals’ molecular form presents a distinct challenge to law enforcement officials. Federal law forbids these slight variations of controlled substances, called analogues, but with more than 400 varieties of chemical cannabinoids available, D.E.A. officials said, and new ones appearing weekly, prosecutors have found cases difficult to prove.

“Cocaine is illegal once you put it on the table,” Mr. Albrecht said. “Here, it’s a different type of trafficker — they look you in the eye across the table and tell you why their stuff is legal, instead of trying to tell you why it wasn’t them.”