A neural suppressant was a drug that inhibited certain types of activity in the nervous system.
In 2376, The Doctor administered neural suppressant to several members of the USS Voyager crew, to block the traumatic memories of the Nakan massacre caused by the neurogenic pulse of the Tarakis memorial monument. (VOY: "Memorial")
Later that year, The Doctor injected Kathryn Janeway, Tuvok, and B'Elanna Torres with neural suppressant as part of a plan to infiltrate a Borg tactical cube. The suppressant allowed them to be assimilated by the Borg without losing their individuality, although Tuvok's wore off prematurely and he eventually succumbed to the Collective. The suppressant also impaired the integration of Borg implants, allowing The Doctor to remove most of them from their bodies after the mission. (VOY: "Unimatrix Zero, Part II")