In addition to routinely administered long-term medication, complex drug regimens of nursing home residents often include as needed or pro re nata (PRN) medication. However, there has been no systematic evaluation of the frequency and concomitants of PRN medication in nursing homes. The main objective of this systematic review was to provide a current assessment of PRN drug use in nursing homes.A systematic literature search was performed. Data were identified from 4 electronic bibliographic databases: MEDLINE, Embase, CINAHL, and Scopus. Studies were included if they reported quantitative data on PRN drug use in nursing home residents.Our search strategy resulted in 484 hits, of which 27 articles satisfied the inclusion criteria. The mean number of PRN drugs ranged between 0.4 and 4.9 per resident with a median of 2.5. The proportion of residents prescribed at least 1 PRN drug was between 48.4% and 97.4% (median = 74.9). Administration of prescribed PRN medication was rather low as the proportion of residents with administered PRN drugs ranged from 28% to 55%. Frequently prescribed PRN drugs were analgesics, laxatives, and sedatives. Advanced age, dementia, a higher number of regularly scheduled medications, and length of stay in the nursing home were associated with higher use of PRN drugs.Although not regularly administered, PRN drug use in nursing home residents should be taken into account as part of complex drug regimens. In that sense, there seems to be an inadequate number of studies reporting on it. When screening tools like the Beers Criteria are adapted, PRN drugs should be included.