If we have a rule of ain tochain achar tochain, then why is medicine prohibited on Shabbos as the reason commonly given to prohibit medicine is that there is tochain during manufacturing.
Any medicine is forbiden on Shabbos even if the particular medicine in question does not require Tochen in its preparation, as we find in Mishnayos Shabbos 109b and 111a. This is because Chazal were Gozer not to take medicine on Shabbos in case a person may come to think that he may do Tochen on Shabbos in order to prepare the medicine. The Gezeirah was obviously only necessary for medicines that do not require Tochen for their preparation, since where a Malachah is necessary no Gezeirah would be needed.
We see from the Mishnayos quoted above that not only medicines which normally involve a Melachah in their preparation are forbidden, but even those that involve no Melachah at all may not be taken on Shabbos. See Shulchan Aruch OC 328:1 and Gra ibid.