Scientists Call for Standardized Reporting of Opioid Side Effects

Drowsiness was reported for 23 treatment cohorts from 18 studies, and also appeared to be dose-related. Rates ranged from 3% to 88%, and the lowest (3% to 50%) occurred in studies that only reported side effects ascribed to opioid use, or relied on spontaneous reporting by patients.

Dry mouth was reported in 12 treatment cohorts from 8 studies, at rates ranging from 1% to 94%. Although 8 cohorts showed rates below 50%, the overall incidence was high compared with other adverse events. Confusion, sweating, and itching were reported in 2, 4, and 6 studies, respectively. Pretreatment with codeine, tramadol, or dextropropoxyphene was not linked to a lower rate of adverse events.  

Urgent Need for Standardized Outcome Measures

The wide variation in side effects could be due to how each study reported and collected the adverse events. Some asked the patients questions about side effects while others only included what patients spontaneously reported, Dr. Oosten pointed out. Some studies only included side effects attributed to opiates, while others reported all symptoms.

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“We are the first to have created such an overview that makes it very clear there really is a lack of good data,” Dr. Oosten said. “That is actually quite shocking.”

Ideally, Dr. Oosten added, more randomized controlled trials of opioid-naïve patients would be conducted to compare the different treatments and their side effects. As that’s not feasible, researchers need to determine the best methods to assess adverse events so that they are reported the same way in every study, Dr. Oosten concluded.

No funding was received for this study. The authors report no conflicts of interest.


  1. Oosten, AW, Oldenmenger, WH, Mathijssen, RHJ, et al. A systematic review of prospective studies reporting adverse events of commonly used opioids for cancer-related pain: A call for the use of standardized outcome measures. J Pain. 2015; 16(10):935-946.
  2. “Opioid Pain Medicines for Cancer Pain” American Cancer Society. Accessed October 12, 2015.