May one cut a medicine in half [for one who may take medicine on Shabbos], such as to achieve a smaller dose for a child and the like?
Yes. The prohibition of Tikkun Keli does not apply by medicine just as it does not apply towards food, and hence one may cut it to a small size for Shabbos use.
Suppository: There is no prohibition in cutting a suppository in half on Shabbos.
 SSH”K 33 footnote 36 in name of Rav SZ”A; Piskeiy Teshuvos 328:39; 322 footnote 20 [New]
 See Michaber 322:4 “Animal food does not contain within it [the prohibition of] fixing a vessel.”; Admur 314:11 “ As animal fodder is soft and does not last, as will be explained in chapter 322 [Halacha 4 ], in a case that one modifies the food to become an independent vessel, being that it is not at all common to initially make a vessel out of food, due to the fact that it does not last long.”
 The reason: Being that this is similar to food which does not contain a cutting prohibition. [SSH”K ibid]