I know two people who have done many, many drugs, acid very much included, but they both strongly agree that datura (which they always refer to as "jimsonweed") is the worst drug experience they have ever had.

Here's an erowid page about it (I'm linking specifically to the "Author's warning" section but I also recommend section 8, "Cautions"): https://www.erowid.org/plants/datura/datura_faq.shtml#5

Tell them it's not actually a drug and it's literally just poison

Psychedelics, weed, caffeine, meth, opiates, etc. all latch onto receptors to give off a high feeling

Deliriants like datura make you "high" by giving you brain damage and destroying its ability to sense the world around you

Similar to how an alcohol high is also just oxygen deprivation and brain damage

have they ever taken datura before?

just this bit from "Psychoactive use" portion of the wikipedia page is enough to put me off:

In Pharmacology and Abuse of Cocaine, Amphetamines, Ecstasy and Related Designer Drugs, Freye asserts, "Few substances have received as many severely negative recreational experience reports as has Datura." The overwhelming majority of those who describe their use of Datura find their experiences extremely unpleasant ─ both mentally and often physically.

plus it's a deliriant, aren't those supposed to be universally kind of a bad time??

why do they want to take it?

https://en.m.wikipedia.org/wiki/EA-3167

If the military engineers drugs to have similar deliriant effects to datura, it's probably not a good idea to voluntarily subject oneself to those effects.

Yeah, doesn't sound like a particularly good idea for them, I seriously do not fuck with delieriants

Ask them if they've ever been around a person suffering from dementia and witnessed an episode of delirium. Ask if it seemed like this person was having a good time.

The drug effects are supposed to be remarkably different.

Just get them to take a lower dose to "get used to it" and they'll give up on their deliriant dreams from the unpleasant body load.

Datura grows wild here as it does in many other places, so I've ventured a little bit down this rabbit hole myself. The first thing I want to communicate is this: At first, I misread your title as "taking someone out using datura" and it still made perfect sense to me. For a split second I thought you were fedposting, but it still made perfect sense. I would consider datura a poison first and a drug hypothetically.

From this article:https://pmc.ncbi.nlm.nih.gov/articles/PMC6220643/

Classic anticholinergic symptoms include mydriasis; dry, flushed skin; hallucinations; agitation; hyperthermia; urinary retention; delayed intestinal motility; tachycardia; and episodes of seizure.3,5,7,8 The mnemonic for anticholinergic symptoms—“blind as a bat, dry as a bone, red as a beet, mad as a hatter, and hot as a hare”—thus applies well to jimson weed poisoning.

I've read a lot of trip reports, so I can explain what this mnemonic means: You can't focus your eyes on anything. You're flushed and badly overheated, but you cannot sweat, piss or shit. You just have to ride it out. Also, you're experiencing hyperreal, vivid hallucinations, and not the fun kind. Acid is fun, acid takes you to the colorful biocurcuitry place of animal existence. Datura sends you right to Benzo Silent Hill. I'll read up and refresh to update this comment, but the hallucinations tend towards the nightmarish, on top of the bad headspace the physical symptoms create. Insects everywhere, shadowy figures and distorted menacing voices with no discernable source, complete loss of place and time, and utterly lifelike apparitions that include conversations with nonexistent people and, oddly, the smoking of phantom cigarettes, even among nonsmokers.

The visions are so lifelike that they can lead to a loss of certainty in reality so deep and traumatic that it persists long after the trip. Almost every trip report I have ever read about Datura has ended in a warning not to try it.

Cardiac and body temperature symptoms:

Patients with anticholinergic poisoning should be observed by using a cardiac monitor because of the risk for tachyarrhythmia from inhibition of vagal effect on the sinoatrial node.9 Propanolol may be used for treating symptomatic tachyarrhythmia; the dosage for adults is 1 mg given intravenously for one minute and repeated every five minutes (maximum dose, 5 mg); the dosage for children is 0.01 to 0.1 mg/kg, (maximum dose, 1 mg).3

Patients also need close observation for hyperpyrexia and convulsions, because either condition can be fatal.

If you take too much (which is easy to do because the minimum effective dose and the lethal dose are uncomfortably close together):

In severe cases in which patients have symptoms of anticholinergic crisis (eg, dysrhythmia, coma, seizures, clinically significant hypertension, or poorly controlled hyperpyrexia), the use of physostigmine is warranted.5