Humalog is an Eli Lilly fast-acting analog insulin also known as insulin lispro. Insulin lispro has been altered from the human insulin molecule at positions #28 and #29 on the B insulin chain. The normal amino acids present at B-#28 and B-#29 are Proline (#B-28) and Lysine (B-#29); insulin lispro reverses this so that Lysine is found at B-#28 and Proline at B-#29.
Inverting the normal Proline-Lysine sequence of human insulin at positions #28 & #29 on the B insulin chain created an analog insulin which has less tendency to form into hexamers (self-association). Keeping the insulin from forming hexamers means that in dimer and monomer form, they are able to be absorbed faster and that means more rapid onset, peak and less duration.
Insulin lispro has been known to "wilt" easily when not constantly refrigerated. Those who have had bad vials report finding hazy or small particles on inspection. Humalog degrades rapidly--it can be working well one day but not the next.
Dr. Nelson of University of California-Davis said in his lecture at the Ohio State Endocrinology Symposium in 2006 that if the short-acting analog insulins have any role in feline and canine diabetes, it is not yet determined.
A study of dogs from 1993-1994 using various fractions of Humalog mixes against human NPH/isophane insulin found only minor differences in the use of Humalog Mix as opposed to NPH/isophane insulin.