Fal-tor-voh was a treatment for a Vulcan neurodegenerative disease, involving a mind meld.
The disease caused the neuropeptide levels in the brain to decrease, and affected the logic capabilities. The treatment could only be performed by a Vulcan who was compatible with the patient, which usually meant a blood relative.
In 2378, Tuvok required fal-tor-voh as he began showing symptoms of the disease. However, none of the Vulcans on USS Voyager were compatible, and he required a relative in the Alpha Quadrant.
In the original timeline, where Voyager spent twenty-three years in the Delta Quadrant, he returned home far too late for the treatment, and by 2404, Tuvok's mind had suffered irreparable damage, leading him to be permanently quartered in a mental hospital. (VOY: "Endgame")
In the new timeline created by Admiral Janeway going back in time to arrange for Voyager to return home after seven years, not twenty-three, Tuvok probably received fal-tor-voh upon his return to the Alpha Quadrant, and survived with all his mental faculties intact.
According to the novel Homecoming, Tuvok did in fact receive the treatment from his son Sek, who came aboard Voyager shortly after its return.