PRN stands for pro re nata, which translates into “as the situation arises” when used as an idiom, contrasts with regular usage and emphasizes situational use only – when the need arises.

It assumes significance when prescribing morphine in the setting of cancer pain; a shift from PRN q 2 hourly dosing to PRN q 4 hourly dosing might mark a shift from the acute phase of opioid titration to the opioid tolerant phase and likely attainment of stable pain control.

Pain score threshold, frequency of use of rescue medication, stable analgesic dose for baseline pain for 48-72 hours and favourable functional status have been used as operational indicators to define stable pain control in the context of cancer pain-related clinical trials.

This term is not to be used in the context of anticipatory prescribing at the end of life.

References

Portenoy, R. K., & Hagen, N. A. (1990). Breakthrough pain: Definition, prevalence and characteristics. Pain, 41(3), 273–281. https://doi.org/10.1016/0304-3959(90)90004-W

Twycross, R., Wilcock, A., & Howard, P. (2020). Palliative care formulary (7th ed.). Palliativedrugs.com Ltd

Caraceni, A., Hanks, G., Kaasa, S., et al. (2012). Use of opioid analgesics in the treatment of cancer pain: Evidence-based recommendations from the EAPC. The Lancet Oncology, 13(2), e58–e68. https://doi.org/10.1016/S1470-2045(12)70040-2

World Health Organization. (1996). Cancer pain relief: With a guide to opioid availability (2nd ed.). World Health Organization.

Farrar, J. T., Young, J. P., LaMoreaux, L., Werth, J. L., & Poole, R. M. (2001). Clinical importance of changes in chronic pain intensity measured on an 11-point numerical pain rating scale. Pain, 94(2), 149–158. https://doi.org/10.1016/S0304-3959(01)00349-9

Bennett, D. S., & Carr, D. B. (2002). Oxycodone in cancer pain. Expert Opinion on Pharmacotherapy, 3(3), 313–324. https://doi.org/10.1517/14656566.3.3.313

Cite as Arora, R. D. (2026). PRN Dosing. In Pall MED Prep. Zenodo. https://doi.org/10.5281/zenodo.18324930