Normally one should just ask one’s posek.
The interesting question has to deal with medicines that are of questionable utility or are designed to take the place of foods (vitamins). The heterim to take treff substances for medicinal purposes certainly wouldn’t apply to a placebo, or perhaps not to a medicine that might not be really all that useful. Guaifenesin, the leading ingredient in non-prescription cough medicines, hasn’t done all that well in studies, and red syrups have to be watched closely (since insect bits are sometimes used as coloring). Tylenol (acetominophen) doesn’t really cure anything, so would there be a heter to take a treff version. Similarly, an antibiotic taken for a virus is worthless. Is there ever a heter to take a vitamin without a hecksher? And if there is a hecksher, does that make it food in terms of brachas and being able to not to have to worry about shailohs an Shabbos.