There’s a ton of Viagra in that tea, and Starlight’s cast a bunch of fertility spells on herself. Trixie gave Star the idea, she’s just trying to be a good shipperwingmare friend.
“Drink it all! Sometimes the tranquilizers settle at the bottom!”
For those wondering, that’s a play on an old MST3k joke, in S1E1, Cave Dwellers, where Ator is drinking from a very-obviously-tinged-with-something-nefarious cup of ‘sacred wine,’ as Tom called it.