A, drug classification commonly used drugs are divided into internal drugs, topical drugs, injectable drugs; do not get confused! Second, the commonly used drug delivery foreign abbreviations qd once daily bid twice daily tid three times daily qid four times daily qh once every hour qm once every morning qn once every night qod once every other day ac before meals pc after meals hs before bed am am pm pm st immediately DC stop prn when needed (long-term) sos when needed R prescription ID intradermal injection H subcutaneous injection IM intramuscular injection IV Intravenous injection