....for those of you who have no idea what's going on, Trellium-D is a mineral which is useful in various forms, but when absorbed into the bloodstream of Vulcans acts as a neurotoxin. In large amounts it causes madness and then death, but in small amounts it causes loss of emotional control. While traversing the Expanse, T'Pol became addicted to Trellium, which the Enterprise was carrying to shield the hull against the effects of spatial anomalies. She eventually almost killed herself trying to get more Trellium and she suffered irreversible damage to her ability to control her emotions.