The Prescription is the eight item in the Ocarina of Time trading sequence. The Broken Goron's Sword is traded for it, and then it must be traded for the Eyeball Frog.
First, Link must acquire the Broken Goron's Sword. After that's done, he must travel to the Death Mountain Trail. At the junction, where Link can either go up a steep slope or go straight to Goron City, he must take the steep path and travel up the ridge, then across the path with volcanic rocks falling. Note that if Volvagia has been defeated the rocks will not fall. After that, Link must climb the wall, and reach the platform at the very top, from where the Great Fairy's Fountain and the Death Mountain Crater can be reached. Instead, Link must run to the left edge, which will trigger a mini-cutscene showing Biggoron standing up. If Link shows him his broken sword, Biggoron will recognize it as his work, but he will also say he cannot fix it, as his eyes are irritated because of the recent eruption. He will, however, take the sword back, and in return he will give Link the Prescription for the World's Finest Eye Drops. The next part of the quest requires Link to travel to Zora's Domain. He must then find King Zora. If Link has already completed the Water Temple, King Zora will probably be already revived. Otherwise, Link must go into the Ice Cavern with a least one empty bottle, scoop up some Blue Fire and then come back to the King's chamber. He must then use the Blue Fire to melt the Red Ice around the King, who will reward Link with a Zora Tunic. Link can now show him the Prescription, and receive the Eyeball Frog in return.