I'd say something with the intensity of weed (relatively low) along with the effects of MDMA. Essentially "MDMA lite"

Marijuana often seems to promote thinking "outside the box" which is probably not what the Brave New World people would want for their population

I think he was inspired by Valium and other benzos. They put people into a docile, low-anxiety state, and they were popular around the time the book was written.

That's also more-or-less consistent with the implied literary reference to the Lotus Eaters, who I think are usually imagined as opium users. Opioids are different but are also downers that reduce anxiety.

Benzos later featured significantly in one of Adam Curtis' film-essays -- maybe Century of the Self, maybe another one. I'd view those films as being in a similar spirit to Brave New World.