Cocaine

Cocaine (benzoylmethylecgonine) (INN) is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic. Specifically, it is a serotonin–norepinephrine–dopamine reuptake inhibitor, which mediates functionality of these neurotransmitters as an exogenous catecholamine transporter ligand. Because of the way it affects the mesolimbic reward pathway, cocaine is addictive.

Cocaine in Atlantis
Athelstan Helms had made a certain study of the noxious alkaloids derived from plants. One particular plant from southern Terranova produced alkaloids that, though a stimulant, had deleterious side effects if used for extended periods of time. This interest led Helms to visit the Atlantean Museum in Hanover, Atlantis when he and his companion James Walton were in Atlantis to investigate a series of murders.

Literary Comment
This is an allusion to Sherlock Holmes, a fictional character that Athelstan Helms is based, being addicted to cocaine. It is not clear in the reference whether Helms is also a user of the "noxious alkaloid".