A single intramuscular injection of 500 or 1000 µg. of hydroxocobalamin to 17 individuals resulted in a 1.8- to 4.1-times higher mean serum vitamin B12 blood level, respectively, 5 hours after injection; a 4.6- and 12.8-times higher level 24 hours after injection; a 2.4- and 5.2-times higher level 72 hours after injection, and a 1.6- and 2.4-times higher level by the 2nd through the 4th week after injection than identical doses of cyanocobalamin administered to 19 individuals. The vitamin B12 blood levels following i.m. administration of 500 or 1000 µg. of hydroxocobalamin were significantly higher during the first 24 and 48 hours, respectively, than they were after a cyanocobalamin zinc-tannate complex given to 17 individuals at identical doses.

After a single i.m. injection of 500 or 1000 µg. of hydroxocobalamin, an average of only 16 per cent and 27 per cent, respectively, of the vitamin B12 was lost in the 72-hour urines, as compared to 60 per cent and 69 per cent, respectively, after identical doses of cyanocobalamin. These differences, again, were highly significant statistically.

The results of these studies give evidence of a slower rate of urinary excretion of hydroxocobalamin as compared to that of cyanocobalamin, and of its ability to build up consistently higher and more prolonged vitamin B12 levels in the blood.

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