<unk>  is a peripheral acting mu-opioid receptor antagonist, and does not cross the blood brain barrier.<unk>  has restricted access through the blood brain barrier because it is a quaternary amine, which carries a positive charge when in a solution and more has polarity with lower lipid solubility than a lot of the opioid agonists used for pain treatment. The peripheral action of<unk>  makes it effective for decreasing the constipating effects of opioids, without interfering with the analgesic effects (of opioids) on the central nervous system. This is the primary characteristic that makes<unk>  behave differently than naltrexone. Furthermore, as<unk>  cannot cross the blood–brain barrier, it does not reverse the pain-killing properties of opioid agonists or cause withdrawal symptoms, but since a small portion of analgesia comes from the peripheral opioid receptors, it can increase pain from inflammatory conditions such as arthritis.
Methylnaltrexone
Dezocine
Bretylium
Butriptyline
Prucalopride
Desomorphine
Naltrexone
Dimethazan