Humulin is synthesized in a special non-disease-producing laboratory strain of Escherichia coli bacteria that has been genetically altered to produce human insulin. Humulin 50/50 is a mixture of 50% Human Insulin Isophane Suspension and 50% Human Insulin Injection (rDNA origin). It is an intermediate-acting insulin combined with the more rapid onset of action of Regular human insulin. The duration of activity may last up to 24 hours following injection. The time course of action of any insulin may vary considerably in different individuals or at different times in the same individual. As with all insulin preparations, the duration of action of Humulin 50/50 is dependent on dose, site of injection, blood supply, temperature, and physical activity. Humulin 50/50 is a sterile suspension and is for subcutaneous injection only. It should not be used intravenously or intramuscularly. The concentration of Humulin 50/50 is 100 units/mL (U-100).