The medicine is in Cianwood City, go to Olivine City then you need the HM Surf. Go to the far West of Olivine city and use surf there, you'll know it's the right place because there will be one of those long green buildings leading to another part of Olivine City. Once in the water start moving towards the South-East and you will arrive at Cianwood City. Go to the Healing Centre and go to the house next to it. In the building, there will be a man behind a till, talk to him to obtain the Secret Potion. Use that on Ampharos.
Hope this helps. :)
Hope this helps. :)
