Go to the lake by the city where crasher wake is and i think after u beat the galactic grunt she gives u the medicine from the shop in johto
Chat with our AI personalities
Cynthia will give it to you.
go to the left of moo moo milk shop, then give the secret potion to the psyduck :)
you have to give them a secret potion that you get from Cynthia
A bit up from the grand lake hotel, you will find Cynthia who gives you the secret potion. Also it will be needed to get to Celestic town, you use it on the Psyduck at route 210. Hope it helped!!
Pokemon league champion Cynthia will give it to you when you run into her at lake valor(near pastoria and sunyshore)