Because opioids are addictive and may result in fatal overdose, most are controlled substances. In 2013, between 28 and 38 million people used opioids illicitly (0.6% to 0.8% of the global population between the ages of 15 and 65). In 2011, an estimated 4 million people in the United States used opioids recreationally or were dependent on them. As of 2015, increased rates of recreational use and addiction were attributed to over-prescription of opioid medications and inexpensive illicit heroin. Conversely, fears about over-prescribing, exaggerated side effects, and addiction from opioids are similarly blamed for under-treatment of pain.