 The effects of morphine can be countered with opioid receptor antagonists such as naloxone and naltrexone; the development of tolerance to morphine may be inhibited by NMDA receptor 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.