Application of a synthetic enkephalin analogue during heroin withdrawal

The effects of 20 mg morphine HCL i.m. and the synthetic metenkephalin analogue FK 33-824 (1.25 mg, i.m.) were compared single blind in 8 addicts undergoing withdrawal from chronic heroin misuse. FK 33-824 was found to reduce withdrawal distress in 6 subjects but was less effective than distress in 6 subjects but was less effective than morphine. The effect of FK 33-824 was described as not morphine-like "an intoxication without euphoria". All 8 subjects preferred morphine to FK 33-824 and five stated that they would refuse to take another dose of FK 33-824. This difference in preference, with regard to the two drugs of substitution, may have been due to side-effects of the enkephalin-like compound: a feeling of oppression in the chest and heaviness in muscles.