, we found six more relevant studies. In total, we included 23 studies with 2648 participants. These studies compared the painkilling ability (benefit) and side effects (harms) of different types of oxycodone to each other or to other strong painkillers. Key results Generally, the studies showed that oxycodone is an equally effective strong painkiller whether taken every six or every hours, and equally effective as other strong pain killers, such as morphine. All the strong painkillers examined in the studies were also associated with a number of unwanted effects, such as vomiting, constipation, and drowsiness. Overall, these do not differ between oxycodone and the other strong painkillers. Hallucinations (where people experience imaginary things, e.g. hearing voices) are much less common as a side effect but we found that they were less likely with oxycodone than within morphine. Overall, we found that the current evidence is comprised of studies that contained small numbers of participants of which many (19%) did not complete the studies. However, since there was very little difference between oxycodone and morphine, more research in this area is unlikely. Studies looking at oxycodone compared to other strong pain killers may be useful. Quality of the evidence We rated the quality of the evidence from studies using four levels: very low, low, moderate, or high. Very low quality evidence means that we are very uncertain about the results. High quality evidence means that we are very confident in the results. Overall the quality of the evidence was low or very low, downgraded because of issues with study quality and size.