"Satan's Children", a novella by Spider Robinson; first published in the anthology New Voices II (George R. R. Martin, ed.) in February 1979. You can read it at www.baen.com.
The drug is called Truth or The Whole Truth (TWT):
"I have synthesized truth in my laboratory. I have distilled it into chemical substance. I have measured it in micrograms, prepared a dozen vectors for its use. It is not that hard to make. And I believe that if its seeds are once sown on this planet, the changes it will make will be the biggest in human history.
"Everything in the world that is founded on lies may die."
[. . . .]
"What's the drug called?" Jill asked.
"The chemical name wouldn't mean a thing to some of the brightest chemists in the world, and I never planned to market it under that. Up until I knew what it was I called it The New Batch, and since then I've taken to calling it TWT. The Whole Truth."
The activists who deploy it are a couple of singers named Zack and Jill:
"What do you wanna do with the stuff? Call the reporters? Stand on Barrington Street and give away samples? Call the heat? Look. We're proposing to unleash truth on the world. I'm willing to take a crack at that, but I'd like to live to see what happens. So I don't want to be connected with it publicly in any way if I can help it. We keep our cover and do our tour—and we sprinkle fairy dust as we go."
"Dose people, you mean?"
"Dose the most visible people we can find, and make damn sure we don't get caught at it. We're supposed to hit nineteen cities in twenty-eight days, in a random pattern that even a computer couldn't figure out. I intend to leave behind us the God-damnedest trail of headlines in history."
"Zack, I don't follow your thinking."
"Okay." He paused, took a deep breath, slowed himself visibly. "Okay . . . considering what we've got here, it behooves me to be honest. I have doubts about this. Heavy doubts. The decision we're making is incredibly arrogant. We're talking about destroying the world, as we know it."
"To hell with the world as we know it, Zack, it stinks. A world of truth has to be better."
"Okay, in my gut something agrees with you. But I'm still not sure. A world of truth may be better—but the period of turmoil while the old world collapses is sure going to squash a lot of people. Nice people. Good people. Jill, something else in my gut suspects that maybe even good people need lies sometimes.
"So I want to hedge my bets. I want to experiment first and see what happens. To do that I have to make another arrogant decision: to dose selected individuals, cold-bloodedly and without giving them a chance, let alone a vote. Wesley experimented himself, with a lab and volunteers and procedure and tests, until he proved to his satisfaction that it was okay to turn this stuff loose. Well, I haven't got any of that—but I have to establish to my satisfaction that it's cool."
Here is the part about the one guru who is immune; he is not specifically identified as Zen but he could be, there are Zen gurus around, among others.
Outside Toronto Zack and Jill made their most spectacular single raid, at the Universal Light and Truth Convocation. It was a kind of week-long spiritual olympics: over a dozen famous gurus, swamis, reverends, Zen masters, Sufis, priests, priestesses and assorted spiritual teachers had gathered with thousands of their followers on a donated hundred-acre pasture to debate theology and sell each other incense, with full media coverage. Zack and Jill walked through the Showdown of the Shamen and between them missed not a one. One committed suicide. One went mad. Four denounced themselves to their followers and fled. Seven denounced themselves to their followers and stayed. Four wept too hard to speak, the one the others called The Fat Boy (although he was middle-aged) bit off his tongue, and exactly one teacher—the old man who had brought few followers and nothing for sale—exhibited no change whatsoever in his manner or behavior but went home very thoughtfully to Tennessee. It is now known that he could have blown the story then and there, for he was a telepath, but he chose not to. The single suicide bothered Jill deeply; but only because she happened to know of and blackly despise that particular holy man, and was dismayed by the pleasure she felt at his death. But Zack challenged her to name one way in which his demise either diminished the world or personally benefited her, and she came tentatively to accept that her pleasure might be legitimate.