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CYP2D6‐Inhibiting Drugs and the Increased Risk of Fall‐Related Injuries Due to Newly Initiated Opioid Treatment – A Swedish, Register‐Based Case‐Crossover Study

It has been shown that newly initiated opioid therapy increases the risk of fall‐related injuries. Yet, it remains to be determined whether drug–drug interactions can affect this negative effect, for instance with drugs inhibiting cytochrome P4502D6 (CYP2D6) that metabolizes codeine and also has a partial effect on tramadol and oxycodone. Our aim was to investigate how CYP2D6‐inhibiting drugs contribute to explaining the risk of fall‐related injuries for newly initiated opioid treatments with codeine, tramadol or oxycodone. Data from a Swedish national case‐cross over study were revisited. This study identified a total of 167,257 fall‐related injuries leading to hospitalization that occurred between 1 May 2006 and 31 December 2009 and linked information about dispensed drugs to them. Use of newly dispensed opioids in the 28 days before fall‐related injury with and without CYP2D6‐inhibiting drugs was compared with an earlier control period. For codeine, there was a two‐times increased risk with concomitant CYP2D6‐inhibiting drug use (OR, 1.76; 95% CI 1.40–2.20) and a three‐times risk increase without (OR, 3.17; 95% CI 2.88–3.50). For tramadol, the risks were doubled when CYP2D6‐inhibiting drugs were used (OR, 2.19; 95% CI 1.84–2.60) and tripled without their use (OR, 3.04; 95% CI 2.82–3.27). The risks were about the same for oxycodone, morphine, fentanyl and buprenorphine irrespective of CYP2D6‐inhibiting drug use. In newly initiated opioid therapies, drug–drug interactions from concomitant use of CYP2D6‐inhibiting drugs are associated with a lower risk of fall‐related injury for codeine and tramadol that undergo metabolism via CYP2D6, but not for other opioids.