You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
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.
cianwood city
The SecretPotion gets rid of the Psyduck on Route 210.
Pokemon league champion Cynthia will give it to you when you run into her at lake valor(near pastoria and sunyshore)
You got to go to the tower in the 6th town and to top and give her the secret potion then she is at the gym
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.
Valor Lakefront
poke where gold near
cianwood city
In Cianwood city there should be a Medical store up north of the gym and the man inside the store will give you the secret potion.
You need the Secret Potion, and you can get it at the Cianawood Pharmacy.
Yes, beating the 7th Gym does not effect getting the secret potion.
Ok, after you get the Secret Potion from Cynthia, proceed on Route 210. That should take you to the Cafe Cabin. Walk to the upper left corner, where the Psyduck are. Press "A" when it asks you if you want to use your secret potion. Note: even after you use the Secret Potion it will still remain in your Key Items section of the bag Hope it helped!!
The SecretPotion gets rid of the Psyduck on Route 210.
to get the secret potion(medicine)you need to go to cianwood