From Bulbapedia, the community-driven Pokémon encyclopedia.
The SecretPotion (Japanese: ひでんのくすり Secret Medicine) is a key item in the Generation II and Generation IV Pokémon games. It is a special kind of medicine, however, its use is restricted, and cannot be used on the player's Pokémon.
In the games
Gold, Silver, and Crystal
The SecretPotion first appeared in Pokémon Gold, Silver, and Crystal. It is only obtainable once, from the pharmacy in Cianwood City. Obtaining the SecretPotion is a compulsory part of the games; it is needed to fight the Olivine Gym Leader, Jasmine. The first time she is encountered she will be with a sick Ampharos in the Glitter Lighthouse, and will not return to the Gym until her Pokémon is healed. The SecretPotion is the only cure for Amphy's illness.
The SecretPotion is given to Jasmine, who will heal Amphy on her own. It will not be returned to the player, and cannot be used to heal his or her own Pokémon.
Diamond, Pearl, and Platinum
The SecretPotion made a second appearance in Pokémon Diamond, Pearl, and Platinum. Like in Generation II, it is an important part of the games' plot, and will heal a flock of Psyduck blocking Route 210 of their headaches. To allow access to Celestic Town, Cynthia, the Champion of Sinnoh's Pokémon League, will give the SecretPotion to the player. How she came about the SecretPotion is unknown, but Jasmine (who kept the SecretPotion in the contemporaneous Johto games) appears in both Sunyshore City and Hearthome City. It is implied through dialog that the events in Johto occur prior to those in Sinnoh.
Unlike in Gold, Silver, and Crystal, the SecretPotion is kept after use in Diamond, Pearl, and Platinum. However, it cannot be used again. Interestingly, the in-Bag description of the item mentions Cianwood City, despite the area being unvisitable in the games.
HeartGold and SoulSilver
Pokémon HeartGold and SoulSilver are the fourth generation remakes of Generation II, and as such the SecretPotion serves the same purpose as in those games. It is obtained in Cianwood City and heals Amphy. Jasmine will take the medicine from the player to do so, and will not return it. Something odd about the picture next to the description is that after use, there will only be two SecretPotions left.
In the anime
Ash, Brock, Misty, and Janina traveled to Cianwood City in Machoke, Machoke Man! to buy the SecretPotion so Sparkle the Ampharos could be healed. Janina took the medicine back on an Express Ferry, while Ash and friends explored the Whirl Islands before returning to the mainland to challenge Jasmine for the Mineral Badge.
The Psyduck Stops Here! is set in the same location that the Psyduck block passage in the games. However, Cynthia does not make an appearance and the SecretPotion is not featured.
In the manga
The SecretPotion was mentioned in two manga series based on the plot of Gold, Silver, and Crystal. Pokémon Adventures and Pokémon Gold & Silver: The Golden Boys both have chapters in which the main characters heal the Olivine City Ampharos.
Later in Pokémon Adventures, SecretPotion appears in Suffering Psyduck where Cynthia uses it to cure the Psyduck from their headache on Route 210.
- Other Potion items have a spray bottle as their in-game Bag sprite, but the SecretPotion features a pouch of pills. This is due to the Japanese name, Secret Medicine, not specifying the design of the medicine and not referencing the standard healing item.