Where do you find medicine for the 6th gym on pokemon silver?

In Pokémon Silver, you can find the medicine for the 6th Gym, which is located in the city of Olivine, by visiting the Olivine Lighthouse. There, you need to speak with Jasmine, the Gym Leader, who is worried about her sick Ampharos. To help her, you must travel to Cianwood City and obtain the Secret Potion from the pharmacy there, which you then bring back to Jasmine to continue your Gym challenge.