- usually combined with a time abbreviation
- q4h prn (every 4 hours at needed)
- Abbreviations has 3 administraiton implacations:
- 1. nurse must wait for given time to elapse (4 hours) before giving the next dose.
2. Once time has elapsed, dose may be given at any time
3. sample times not given because nurse does not know when the patient will need the drug