My understanding is that the herb has to be eaten periodically in order for its effects on the eater's physiology to continue. However, only the sitting Black Panther -- presently Shuri -- is normally allowed to partake. There have been exceptions, though. Kaspar Cole for example, al though I think he was given it by T'Challa's half brother Hunter who wasn't one to always adhere to the customs. My guess is that without partaking of the herb on a regular basis because of losing the throne, its effects on T'Challa wore off.
For folks outside of the T'Challa's family line, the herb is lethal. Kasper Kole got a synthesized version of the heart shaped herb from Eric Killmonger. Eric (T'Challa's sworn enemy) did it to anger the Tribal Leaders. T'Challa sort of manipulated the situation to have Eric do it knowing that he would because of his disdain for Wakanda tribal tradition.
It was a tightly woven story of manipulation and end games that lead to Kasper getting his heightened abilities. Of course T'Challa forced him to go through a rite of passage before allowing him to assume the mantle of the White Tiger.
Much respect to Priest for doing his thing on this and other stories in the BP mythos. Smh at Hudlin for unraveling such intelligent storytelling in favor of his Blaxploitation version of BP.