Assuming your Rabbi already permits you to take medicine on Shabbos for a serious chronic disease: If your doctor increases your dosage by half a pill, and the pill is already pre-scored to make it easy to split into two, can you split the pill on Shabbos, or must you do it before? What if you forgot until Shabbos had already started? Would splitting the pill be like grinding? If you pre-split some pills, what is the best way to find a whole and half pill from the bottle and avoid borer (the melacha of selecting)?
Shmiras Shabbos Kehilchoso (Chapter 33:4) writes that it is permitted to cut a tablet into two equal halves in order to use only half.
In the footnotes (footnote 30) he quotes this in the name of Rabbi Shlomo Zalman Auerbach and explains and there is not issue of the prohibition of "Mechataich" (measuring), because the Melocha of "Mechataich" only applies were he wants to use the cut item and not if he wishes to swallow it. In this regard, medicine is considered like food. Furthermore, since he is measuring not for the purpose of having a specific size tablet, but is rather concerned about the volume, this is not considered "measuring" in terms of "Mechataich".
There would certainly not be any issue of "Tochein" here because "אין טוחן אחר טוחן" - there is no problem of grinding something that was already previously ground.
However, if the tablet has words across - for example, the name of the medicine - breaking it in half may be an issue of "Mocheik" (erasing letters).
Most probably there would be no problem of "Borer" in picking out the half tablets. Shulchan Aruch (OC 319:3) rules that "Borer" only applies when separating two different types of items, but not when picking from the same type. Big and small items of the same type are considered one type. [Regardless, as long as you pick out by hand the one you want for immediate consumption it is not a problem of Borer].
Obviously, CYLOR for any matters of practical Halacha.