Sometimes the effectiveness of a drug depends on how our bodies process it.  A lot of drugs are inactive until our liver enzymes transform them into an active form. Opioids (Vicodin, Percocet, OxyContin) work in this manner.  Someone whose body has little or no ability to process opioids into their active form receives very little or no pain relief. They are also more likely to take a higher dose than was prescribed in their quest for pain management.