It's not unusual for people living with chronic pain to also be dealing with some depression and/or anxiety.  But if you're taking an opioid like oxycodone for the pain and also taking an herbal supplement containing St. John's Wort, you may unknowingly be reducing the effectiveness of your pain medication.  

A small study in Finland found that when St. John's Wort and oxycodone were taken together, the plasma concentration of oxycodone decreased by 50 percent and its half-life (the time it takes for half the drug to be elimitated from the body) was shortened by 27 percent. 

The reason for the significant decrease in oxycodone's effectiveness may lie in the fact that St. John’s wort is a well-known to induce CYP450 liver enzymes, which play an important role in the metabolism of many opioids.  Although oxycodone was the only opioid tested, it would be logical to think that other opioids which are metabolized through the CYP450 pathways might be similarly affected.  (Three opioids that are not metabolized via CYP450 are morphine, hydromorphone and oxymorphone.) 

St. John's Wort, a popular herbal supplement used for depression and anxiety, is found in many over-the-counter remedies.  It comes from the plant species Hypericum perforatum and is also known as Tipton's Weed, Chase-devil, or Klamath weed. 

If your pain medication is not working as well as it should and you are taking any kind of supplements, it would be a good idea to check the ingredients of the supplements to see if they contain St. John's Wort. 

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Sources:
Nieminen, Tuija H., et al. St John’s wort greatly reduces the concentrations of oral oxycodone. European Journal of Pain. Volume 14, Issue 8, 854-859. September 2010.
Leavitt, SB. St. John's Wort may thwart opioid analgesia. Pain-Topics.org. August 28, 2010.