In the past few months, it's become nearly impossible to buy Ketaset in New York City's underground drug market. Made by Fort Dodge, an Iowa-based pharmaceutical firm, Ketaset is a brand of ketamine, a compound that blocks certain neuroreceptors, causing hallucinations in high doses and, in lower doses, a fuzzy dissociation - like the warmth of a couple of Jim Beams. Legally, it's used as an anesthetic. Illegally, one snorts ketamine because the fuzziness lasts half an hour and doesn't produce bourbon's four-Advil hangover.

Ketaset's scarcity dates back to August 1999, when the U.S. Drug Enforcement Administration, acting on preliminary evidence that ketamine may lead to dependence, subjected its legal purveyors to strict security rules. But K, as users call it, had already won so many devotees that traffickers were smuggling off-label brands from Mexico. Today Manhattan dealers sell a gram of K for $80, up 100% from 1998.

The recent history of K limns a well-established law of recreational drug use: once users find a substance they like, they will snort or shoot or drop whatever version is available, whatever the cost. Which is why you must look to the market to understand the future of drugs used for anything other than doctor-approved healing. That market can be divided into three groups: the partyers, who just want to have fun (and who sometimes become addicts); the shrinks and shamans, who believe drugs can expand your consciousness; and the scientists, who suspect that illegal drugs - or their chemical cousins - may have marketable legal uses. These groups are distinct but tightly linked: scientific research leads to new drugs, which shamans discover and use in their quests, which often turn out to be as much fun as spiritual. The use of drugs in party settings eventually leads to government crackdowns.

But as a rule, the partyers don't pursue the new drugs; they tend to find a potion and stick with it, sometimes until it kills them. Today's popular party drugs are derived from ancient medicinal herbs: marijuana from hemp, cocaine from coca leaf, prescription painkillers from poppies. It's the shamans who aggressively seek out new substances. Recent additions to the U.S. market include ayahuasco, a plant long used in religious ceremonies in Brazil for its mind-manipulating qualities, and Salvia divinorum, a soft-leaved plant native to Mexico that is chewed or smoked for hallucinogenic effects.

New compounds do occasionally come from underground drug labs or, like MDMA (ecstasy), are rediscovered after years of being ignored in scientific literature. In this world, no one is held in greater esteem than Alexander Shulgin.

Shulgin is a biochemist who once studied psychedelics for Dow Chemical. Now 75, Shulgin has synthesized hundreds of compounds in the smelly lab in the woods behind his California home. He and his wife Ann, a therapist, have published two books that are the bibles of underground drug research: PIHKAL (Phenethylamines I Have Known and Loved) and TIHKAL (Tryptamines I Have Known and Loved). Many of the drugs that have emerged from underground labs can be traced to well-thumbed copies of the Shulgins' books.

It was they who helped popularize MDMA - a signal event in the history of recreational drugs. Ecstasy is easily the biggest advance since LSD. It changed not only the party world but the shaman world, where it was used by psychologists who believed it had therapeutic value. Since MDMA was banned in 1986, scientists have looked for compounds that have the same effects without damaging neurotransmitters, as MDMA can. They haven't had much success.

So today's nonmedical drug research tends to focus on new uses for old substances. That effort is led by Richard Doblin, who runs the Multidisciplinary Association for Psychedelic Studies out of his Belmont, Mass., home. Founded the year MDMA was outlawed, the association uses its $530,000 yearly budget to assist scientists who, with government permission, study the risks and benefits of a wide variety of nonmedical uses for psychedelic drugs and marijuana. Such research is highly political, however, and it can take years for a research protocol to be approved.

The new drugs that appear on the market usually do so after underground chemists read scientific papers and decide to cook something up. Scientists studying how cocaine works in the brain, for example, have developed a version 100 times more powerful. The recipe is available in academic journals, waiting to be exploited.