Gold received the Secret Potion from Chuck in The Secret Of The Fighting Type Pokémon. It was then sent to Olivine City to heal Amphy. According to Chuck, the pharmacy that sold the medicine closed down ages ago, making his sample of it the only remaining one.
Red and his Pokémon went to Cianwood City in Light Up the Lighthouse!! in order to get a Secret Potion from a pharmacy. When they went in the pharmacy, the owner gave it to them after telling them a tale. While at sea, Clefairy lost it, but took one of the pills prior to it getting lost. When Clefairy later ate the pill when their ship was approaching the unlighted Olivine Harbor, he started to glow, saving the ship from crashing.
Other Potion items have a spray bottle as their in-game Bag sprite, but the Secret Potion 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.