i think you go to the psyduck next to the moomoo milk place then the Pokemon league champ will show up and give you it
Chat with our AI personalities
you go down to the house next to the Pokemon center and inside is this guy, talk to him and he will give a secret potion.
Cynthia will give it to you.
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
cianwood city
The SecretPotion gets rid of the Psyduck on Route 210.