<unk> s are substances that act on<unk>  receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for<unk>  use disorder, reversing<unk>  overdose, suppressing cough, as well as for executions in the United States. Extremely potent<unk> s such as carfentanil are approved only for veterinary use.<unk> s are also frequently used non-medically for their euphoric effects or to prevent withdrawal. Side effects of<unk> s may include itchiness, sedation, nausea, respiratory depression, constipation, and euphoria. Long-term use can cause tolerance, meaning that increased doses are required to achieve the same effect, and physical dependence, meaning that abruptly discontinuing the drug leads to unpleasant withdrawal symptoms. The euphoria attracts recreational use, and frequent, escalating recreational use of<unk> s typically results in addiction. An overdose or concurrent use with other depressant drugs like benzodiazepines commonly results in death from respiratory depression.<unk> s act by binding to<unk>  receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract. These receptors mediate both the psychoactive and the somatic effects of<unk> s.<unk>  drugs include partial agonists, like the anti-diarrhea drug loperamide and antagonists like naloxegol for<unk> -induced constipation, which do not cross the blood-brain barrier, but can displace other<unk> s from binding to those receptors. Because<unk> s are addictive and may result in fatal overdose, most are controlled substances. In 2013, between 28 and 38 million people used<unk> s 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<unk> s recreationally or were dependent on them. As of 2015, increased rates of recreational use and addiction are attributed to over-prescription of<unk>  medications and inexpensive illicit heroin. Conversely, fears about overprescribing, exaggerated side effects, and addiction from<unk> s are similarly blamed for under-treatment of pain. Educational video on<unk>  dependence.
Opioid
Heroin
Synthetic cannabinoids
Nicotine
Phencyclidine
Cocaine 
Cocaine
Benzphetamine