The effects of morphine can be countered with opioid antagonists such as naloxone and naltrexone; the development of tolerance to morphine may be inhibited by NMDA antagonists such as ketamine or dextromethorphan. The rotation of morphine with chemically dissimilar opioids in the long-term treatment of pain will slow down the growth of tolerance in the longer run, particularly agents known to have significantly incomplete cross-tolerance with morphine such as levorphanol, ketobemidone, piritramide, and methadone and its derivatives; all of these drugs also have NMDA antagonist properties. It is believed that the strong opioid with the most incomplete cross-tolerance with morphine is either methadone or dextromoramide... The upper molecular weight limit for a small molecule is approximately 900 Daltons, which allows for the possibility to rapidly diffuse across cell membranes so that they can reach intracellular sites of action.[1][2] In addition, this molecular weight cutoff is a..