Which of these notations is on the Institute for Safe Medication Practice's list of error-prone abbreviations, symbols, and dose designations?

"HS" may be interpreted to mean "half strength" or "bedtime"; as such, it is most appropriate to write out the term intended. "PO" is one of the few abbreviations supported by the Institute for Safe Medication Practice. Other options for "PO" include "by mouth" or "orally," but not "per os."