Oral ingestion of narcotic analgesics by rats.

Rats drank solutions of narcotic analgesics as the only drinking fluid available in their cages. Relative to their daily water intake before the drug solutions were introduced, the rats drank more etonitazene solution, less methadone solution, and about the same volume of morphine, meperidine and levorphanol solution as water, although some rats would not drink the higher concentrations of morphine (1.0 mg/ml), levorphanol (1.0 mg/ml) and methadone (1.0 mg/ml). When naloxone (1.0 mg/kg) was administered after 12 days of drinking the drug solutions, the severity of the abstinence syndromes based on scoring of symptoms and on weight loss was morphine greater than or equal to etonitazine greater than levorphanol greater than meperidine greater than methadone. The animals showing the most severe syndrome after naloxone usually consumed more drug solution than usual during the next 24 hours, so that body weights were normal at the end of the 24-hour period. Nonprecipitated abstinence was also studied in these rats by replacing the drug solutions with water 3 to 5 days after the naloxone injection. Rats that had been drinking morphine, etonitazene and levorphanol lost weight for 2 or 3 days and then began to regain their lost weight. In contrast, rats which had been switched from methadone and meperidine solutions to water gained weight rapidly. Morphine and etonitazene drinkers which had been switched to water drank a lesser volume of water than they had been drinking of drug solution whereas some ex-methadone drinkers drank more water.