Because it carries with it a substantial risk of hypoglycemia,  giving insulin intramuscularly should only be done with guidance from a medical professional. Insulins which are not soluble are able to be used intramuscularly, unlike intravenously, which requires soluble insulins only.
Intramuscular insulin injection is a technique used for both pets and people in an effort to hasten onset, compared to the usual technique of injecting subcutaneously.  It's often referred to medically by the abbreviation IM. 
Because absorption of subcutaneous insulin can be altered by dehydration (mainly slower than normal), intramuscular insulin injections can be used for cases involving dehydration and/or ketoacidosis. 
- ↑ Thow JC, Johnson AB, Fulcher G, Home PD. (1990). Different Absorption of Isophane (NPH) Insulin From Subcutaneous and Intramuscular Sites. Diabetic Medicine.
- ↑ Insulin Treatment-Needle and Syringe. Patients Up To Date.
- ↑ Questions About Insulin. Diabetes-World Mailing List.
- ↑ Heinemann, Lutz (January 2008). Variability of Insulin Action:Does It Matter?-page 40 (4 of 9). Insulin Journal.
- ↑ Herrtage, Michael (2009). New Strategies in the Management of Canine Diabetes Mellitus. WSAVA.
- Variation in Absorption of NPH Insulin Due to Intramuscular Injection Diabetes Care-1990