The other day, I wasn’t thinking and pushed the Humalog (I had just drawn) into the NPH (relatively full vial). I discarded the NPH and opened a new vial. Would a small amount Humalog make the NPH unusable? If so, is it due to a chemical reaction that “ruins” the NPH or just the effect of diluting the NPH?

Actually, a small amount of short-acting insulin such as

Humalog

can “contaminate” a vial of NPH with little effect upon the longer-acting insulin so with the example that you cited, there would be little reason to worry. However, the converse is more worrisome. A small amount of a longer-acting insulin can corrupt a bottle of short-acting insulin.

NPH is Regular insulin to which a special additive (“protamine”) has been added. The addition of protamine extends the duration of time that the insulin is absorbed and has its effect. Therefore, if you add NPH to short-acting insulin, the result is an insulin that is no longer short-acting. Adding a small amount of short-acting insulin to a longer-acting insulin does not really corrupt the onset of the longer-lasting insulin in any appreciable way.

DS