THE road has not been ultrasmooth for the wilder young luminaries of the 1960s--paritcularly those with a strong interest in Pharmacology. They have been to jail. They have turned to God. They have become insurance salemen and stockbrokers.

And yet here's Dr. Andrew Weil, still at it, offering no apologies and showing no obvious signs of wear, two decades after he cut a memorable swath through Harvard and Harvard Medical School with his extravagant pranks, his roly-poly profile and his highly empirical researches into the effects of marijuana and mescaline on volunteer members of the university community.

He has shed some weight over the years and acquired a bushy gray-black beard, and he has become a certifiably reputable scientist--a research associate in ethnopharmacology at the Harvard Botanical Museum, an adjunct professor of addiction studies at the University of Arizona and so forth.

And he wants to firmly dissociate himself from his former avocation as a perpetrator of hoaxes, fearing damage to his scientific reputation if the world knew about: a. the letter he once wrote on the stationery of New York City Mayor John V. Lindsay, accepting Princeton University's nonexistent offer of an honorary degree; or b. that phony radio broadcast that had a number of his Harvard classmates convinced nuclear holocaust was imminent; or c. the time he stole the Harvard Lampoon's treasured Ibis and sent it around the world in the columns of the Crimson, pursued by a tireless detective firm known as Find-a-Bird Inc.

But otherwise he remains, at 40, the same amiable bear of a man with the same bubbling laugh, the same low tolerance for pretension and pomposity, and the same life's work: the study of how and why human beings the world over are forever trying to change what's happening inside their brains.

"I'm willing to try anything once--if I'm fairly sure it won't kill me," says Weil, through whose hardy system has passed almost every psycho-active substance known to man, from marijuana to PCP to yoco. (More about yoco later--and about yage and yopo, while we're at it.)

Weil was never a pharmacological proselytizer a la Timothy Leary. He was, in fact, the driving force behind a series of Crimson articles that helped get Leary and his associate Richard Alpert removed from the Harvard faculty--an episode that forms a subplot in the just-published "Splendor and Misery: a Novel of Harvard" by Faye Levine, who writes of her Weil-based character, Thaddeus P. Foote, that "uniquely in the university, he seemed to have one foot in the drug world and one foot out of it."

Weil's own drug studies haven't landed him in trouble with the law. "I have generally found the drug authorities to be very helpful," he says.

But his ideas are hardly of the government-approved type. His new book, "Chocolate to Morphine: Understanding Mind-Active Drugs" (cowritten with Winifred Rosen), aims to counter the influence of "fear and prejudice" on the young experimental set, and includes these eyebrow-raising observations:

"You are less likely to encounter problems if you take dilute forms of natural drugs by mouth . . . "

"Any effort to eliminate drugs from society is doomed to failure."

"Remember that wanting to feel high is not a symptom of mental illness or an unhealthy need to escape from reality. It is normal to want to vary your consciousness."

At the core of the Weil philosophy is his belief that most drugs aren't inherently good or bad, but there are good and bad ways of using them. Which brings us back to yoco--and to the Ingano Indians, one of six Amazon tribes that like to start the day with an extract of this bitter, caffeine-rich jungle vine.

"You wake up among these people to the sound of yoco being scraped just as the sky is getting light," says Weil--speaking from experience, as always.

"It was very hot and humid in this area and there wasn't much to do," he recalls, "and I would lie around in a stupor in my hammock most of the day. And one time in the afternoon I thought, 'Gee, it would be wonderful to have some yoco,' and I began asking this guy to make me some. And finally he just said in Spanish, 'That's not our way of doing things.' Once the yoco is put away for the day, you don't use it again till the following morning."

This struck Weil as a sound way of limiting the consumption of a stimulant like caffeine, in marked contrast to the American practice of gulping coffee through the day.

"The energy that you feel when you take a stimulant is not some magical gift that drops from heaven," he says. "It's your own chemical energy that your nervous system has stored in the form of neurotransmitters. Stimulant drugs just force your nervous system to give up that energy sooner than they would normally, and the consequence of that is that when the drug wears off, you're left with less energy . . . If you take more of the stimulant at that point--to get out of that trough--that is the way that you fly very quickly into a dependent relationship with that drug."

Many primitive tribes also display startling sophistication, according to Weil, in their recipes and modes of preparation. He cites the common South American practice of mixing a brew derived from the psychedelic plant yage, or ayahuasca, with the leaf of a plant containing another psychedelic substance called DMT, or dymethyltryptamene. "The Indians say they put the leaf in to 'make the visions brighter,' " he says. But when anthropologists first learned of this, they assumed the DMT had no real physical effect, because the human stomach was known to produce an enzyme rendering DMT inert when consumed orally.

"Well, lo and behold," Weil announces with a grin, "it turns out that the drugs in yage inactivate the enzyme in the stomach, so that when you mix the two together, you get an orally active form of DMT."

In the industrial world, on the other hand, Weil argues that both legal and illegal drug use have been made riskier by an insistence on trying to "pull out" of every mind-active substance its concentrated chemical essence. "We have come up with forms of drugs that are much more toxic, that are much more abusable," he says, "because they can be put into the body and the bloodstream by much more direct ways--by snorting and shooting."

Not that any part of the world has an exclusive monopoly on these practices. One of the most stressful drug experiences of Weil's life involved yopo, a finely powdered snuff favored by certain Amazon tribes who "blast it into each other's noses through long tubes, very forcefully. The feeling is like daggers going into your brain, followed by a very short, violent intoxication with a lot of visions and a great many physiological upsets.

"It's not very pleasant," he adds helpfully.

But by and large, Weil's travels have given him--or failed to take away--a certain abiding distrust for the so-called advances of western science, particularly when it presumes to preach to heathens. On certain Pacific islands, says Weil, the ritual use of kava root (a pepper-plant-derived sedative) was eliminated by Christian missionaries, with "rampant alcoholism" as a result. "And those people have no tradition or cultural support for using alcohol."

Of late, the World Health Organization has been campaigning against the use of a stimulant leaf called qat, which, according to Weil, is the "drug of choice" of the predominantly Moslem lower classes in Ethiopia, while the Christian upper classes prefer alcohol. "So the dominant population is delighted to invite the U.N. in," he says, "to stamp out qat on the grounds that it makes people shiftless and amotivated. The usual arguments."

Weil views drugs as "only one route into certain experiences that people seem to need." Hence, in one of the few recommendations likely to please old-fashioned parents who read his new book, he counsels young people to consider getting their highs without putting chemicals into themselves.

He is also deeply interested in the broad question of why drugs and certain nonchemical experiences seem to do such similar work in the human system. It begins to look, he says, "as if almost every external drug has some kind of internal analogue"--the one major apparent exception being marijuana. "We certainly make counterparts of psychedelics, we make counterparts of downers, of stimulants, of antidepressants," says Weil. "And it may be that when we do activities that change our moods, those changes are mediated by changes in the production or the activity of internal drugs--neurochemicals.

"That also raises a very curious question of why it is that plants produce chemicals that resemble the structure or activity of molecules that we have in our brain. Why have a poppy plant and the human brain evolved substances that have the same effect?"

Among the more bizarre nonchemical highs he has studied is that generated by eyewitnessing a solar eclipse.

An eclipse, he says, produces a "total distortion of sensory input because you see the world in a light you never otherwise see. People get very high afterward. And interestingly, the social attitudes toward eclipses are very parallel to the attitudes toward drugs. The authorities tell you that they're dangerous--that you'll go blind if you look at the sun, that you should stay in your houses and watch them on television."

But don't people go blind?

"The moment the sun is covered," says Weil, "you can stare at it with your bare eyes and nothing happens"--nothing destructive, that is. His last eclipse was three years ago in Winnepeg, Canada, where, he laments, there were "horrendous warnings" from the medical authorities. "I knew one family that hung blankets over the windows of the house on the side facing the sun, and they all gathered in the bedroom to watch the televised version of the eclipse.

"And these are the same people," he adds with a roll of his eyes, "that laugh about silly beliefs in Indians, who think dragons are eating the sun!"