A form of mind meld with a compatible Vulcan, usually a family member, which is the only cure for the degenerative neurological condition that Tuvok was concealing in Endgame.
Fal-tor-voh is the treatment for a progressive neurological disease that can afflict Vulcans, gradually destroying their mental discipline. It is not a drug but a mind meld, and it works only with another Vulcan whose mind is sufficiently compatible, which in practice means a blood relative.
The Voyager finale Endgame reveals that Tuvok has the condition. In 2378 it is at an early stage, known only to him and the Doctor, who can slow it with medication but cannot stop it. In the future from which Admiral Janeway travels, Voyager took a further sixteen years to reach home, and by then it was far too late: Tuvok is shown in 2404 confined to an institution.
Knowing that he would have recovered had the ship returned sooner is one of the admiral's main reasons for changing history. In the new timeline Voyager reaches Earth in 2378, with Tuvok's family within reach.