there is a pharmacy in cianwood, if you look about you should find it, when you go into the building talk to the guy behind the counter and he will give you the medichine
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cianwood city
after talking to jasmine go to cianwood and go to the south most house go and talk to the man at the desk he should give you the medicine
In Cianwood City you receive the medicine from the medicine shop. To give the medicine to the gym leader you have to go to the top of the lighthouse in olivine, which is where she will be. She will then return to her gym where she will battle you. Be careful - her Pokemon (2 magnemites and a Steelix) can be tough! Good luck! :)
Surf to Cianwood City, there is a pharmacy there and the guy inside will give you secret medicine.
if you mean the medicine for 'amphy' in the light house, go to the medicine man in cianwood city he will give you a 'secret potion'