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Oxytrex is an investigational drug currently in clinical trials. It combines the well known opioid agonist oxycodone in the treatment of pain with an ultra-low dose of naltrexone, an opioid antagonist. Naltrexone can have markedly different pharmacokinetics at ultra-low doses which may include, the attenuation of a pre-established opioid tolerance, the prevention of tolerance from the oxycodone in the formulation, and the possible amplification of oxycodone. It is being developed by Pain Therapeutics, Inc. for the treatment of severe chronic pain.

A phase III clinical trial has shown that the combination of ULD (ultra-low dose) naltrexone with oxycodone has been nearly as effective in providing pain relief while causing less physical dependence than oxycodone alone. The trial has been criticized for its confounding and limiting factors which include the huge dropout rate (54%), lack of demographic stratification of pain intensity amongst the study arms, and that patients received varying amounts of oxycodone which would influence their overall dependence. The lack of an on-board break-through-pain agent also hurts their external validity and likely contributed to their high dropout rate.