Right, she took it to see what it was like so clearly she didn’t need it but he appeared to actually need it. He was going to lose to that other guy if he didn’t juice.I'd guess that V has something addictive in it, based on the way Popclaw talked about it to A-Train, and we also see that in the way his behavior changed when we knew he was on it and the way Popclaw acted when she was on it. Maybe they don't necessarily need to keep taking it to maintain their abilities, but judging by how it enhanced A-Train's speed, it appears that it will continue enhancing but at a horrible price.
Maybe he doesn’t need it but without it he’s not the fastest so he’s taking it to boost his power?
Not sure they’ll ever even cover it.