Viewing the Vibe through a Bright Glass Pipe: Narcotics and Delta Green.
Mind altering substances, both legal and illegal, are innately well suited to the Delta Green setting. I would caution that I'd consider maturity and some thought to be necessary before using such substances in a Night at the Opera, especially if you intend the Agents to make use of them. Then again, you're playing Delta Green, not D&D or Toon, so I'd imagine you've got those requisites. Damn, now I'm imagining a version of DG set in a Yellow-Submarine-ish Dreamlands.
Well, I won't be giving stats or details of specific drugs- this isn't a D20 supplement and I'm not being paid by the word. Or at all- but will mainly focus on "recreational" drugs. Instead I'll just give a quick run through of the three main types of drugs, and examples. Do note that I'm no doctor. Hell, I'm not even old enough to drink in the nation where DG is set. This is just a quick reminder. By effect and legality:
Hallucinogens: Warp your senses as well as your other brain processes. Varying from LSD/acid, which is responsible for the stereotypical trip complete with plaid patterned rabbits, to the more legal (in UK) psychoactive fungi. Many other drugs have some hallucinogenic effects.
Stimulants: Stimulate the metabolism and flight-or-fight responses. Heart racing. Tend to lead to pschyosis and sleep deprivation. Amphetamines, caffeine, ecstasy and fear.
Depressants: Ranging from clinical sedatives such as diazepan, PCP and ketamine (anesthetics) to plain old fashioned malt whiskey.
Now for the meat of this little piece of nonsense: narcotics and your adventures thwarting the mythos. Narcotics may enter the story as plot devices in the following manners (and more):
a) Criminally Supplied Narcotics. The best term I could think of, this is when your DEA agents bust a sedan with a dash board full of Colombian. Perhaps the driver is off to buy a mythos tome with it. Or perhaps he is about to be jumped by Karotechia thugs looking for a quick buck, who decide the risk incurred by killing a DEA agent is still nothing compared to the cash. For variants, let the PCs plant drugs on cultist suspects. Or have the FBI/NSA/DEA etc come across something quite unexpected- chest full of engraved human bones, a Yithian device, etc- when they flag down a suspect vehicle.
Remember that the pipelines used for drugs are also used for other illegal things- people trafficking, weapons, Mythos items (duh!). Also that it is a prime opportunity for some badge flashing by the Agents- for once, they might not even need to BS the jurisdiction. The drug Reverb, from DG:Countdown, blurs the line between this category and-
b) Mythos Drugs. The Plutonian Drug, Space mead, Reverb etc, all coming from beyond mortal ken but distinctly interactive with mortal physiology. Come up with the wacky effects here- from being hurled into the past to spontaneous summoning (of course, NOT binding :D ) of mythos beasties to a long conversation with Stephen Alzis about the NFL playoffs (his predictions were hideously unlikely and implausible. Doesn't mean they're ALL not going to happen) in a hallucination. Such drugs may well originate off Earth or in the Dreamlands (ghoul dealers?). Many will have non-recreational uses. I seem to remember there's a transcript of an online campaign on the DG page dealing with some kind of mythos drug being peddled by [MJ12??] into a ghetto in New York.
c)i] Open Your Mind, Man! Using drugs to give SAN checks, with corresponding knowledge of the Mythos. Perhaps your character has his "creativity" triggered, seeing a recurring hallucination of a titanic city of submarine edifices. Perhaps they have a stilted series of impressions of the cultist's base, centered around their statues and idols, allowing not just an impression of the layout of the place, but of the nameless truths and monstrosities the cultists worship. Perhaps the character simply stares at their hands for fifty minutes, before deciding that SOMETHING must have designed the opposable thumb, and carried out tunings and check ups over the course of history. Of course, the cultists may well use drugs in this way as much as the Agents.
Remember that under the influence of narcotics, many perceptions and intuitive guesses may be made which seem wild by the standards of the sober mind. Speaking personally, I've come up with stupid but crazily off-beam ideas after an evening of absinthe. Things get twisted around till their at ninety degrees and inside out, and sometimes they look like they make sense that way. In DG, they may well…
c)ii] Affect the mood of the game. Adding mood altering substances will obviously alter the tone of the scene. If you want something surreal and disconcerting, have an Agent try whatever was in the artist's bong's bowl (in Night Floors, perhaps?). If you want paranoia, perhaps a nasty trip or an episode of amphetamine psychosis will help when the Agent's boss tells him the "NSA" wants to talk to him about what he did on his "hunting holiday" last month". Essentially, use it to distort the characters' perceptions, moods, actions. It's a nice cheat if you have to take their control away for a scene. Alternatively, have an NPC out of their heads for a scene-setter. See movies for good ideas. Even the most apparently surreal and humor-filled can be terrifying if removed from context. Play a pair of stoned teenagers caught by the Agents in a derelict house as possible mind controlled zombies/cultists- and unable to coherently defend themselves. Or look at Fear and Loathing in Las Vegas, and imagine how terrifying it would be to sit next to Dr Gonzo on a bus ride.
Remember that the loss of self control, coordination and higher thought is going to be very scary- and possibly exhilarating, especially if deliberately induced. A particularly odd trip (and what other kind is available in this game?) can easily inflict SAN rolls.