it is used for the the psyduck blocking your way in solaceon town
First, go to the top of the lighthouse (not by the elevator, but by the stairs) to meet Jasmine and be told that you need a secret potion from Cianwood. Go to the Cianwood Pharmacy to get the potion. Go back to the top of the lighthouse and talk to Jasmine to give the secret potion to her.
You have to get the secret potion from Cynthia.
In Pok_mon Flora Sky, you can obtain the secret potion from Cynthia while you are in the Honey Village. You will reach Honey Village after you go through Small Town, Mystic Town and Habutest Town.
what red haired guy and what posion?!
Surf there using the HM u obtained from the Ecruteak dance club when you arrive in OLIVINE city head south. The blue-roofed building right next to the the Pokemon centre is the pharmicist and he will give u 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!!
There is no legitimate secret potion cheat for Pokémon Diamond, Pearl and Platinum version. One way to do this would be to use an Action Replay.
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!!
easy
Use A SECRET POTION at Junkyard Keeper !The Black HOLe
go to the left of moo moo milk shop, then give the secret potion to the psyduck :)
The Psyduck are at the northern part of Route 210, standing in the way of the path. These are the Psyduck you will need to use the Secret Potion on.
from Cynthia
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
get a secret potion or talk to them or use action replay
The Secret Potion is a Key Item which can not be discarded, sold or used until the required event with the troubled Psyducks.
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.