I suspect that the recommendation to inject in the leg was made out of an abundance of caution to avoid injection site sarcoma (ISS), while the recommendation to inject in the scruff was made from a combination of factors:
- It's the easiest site for an owner to give an injection
- There's no proof that insulin can cause ISS
- Some vets still aren't aware of ISS
What Is Injection Site Sarcoma?
ISS, previously called Vaccine Associated Sarcoma (VAS) is a particularly invasive form of cancer that is believed to form at any injection site that may have some inflammation (Woodward 2011). However, the only proven causes of ISS are prior administration of killed, adjuvanted rabies or leukemia vaccine (Wilcock 2012).
Adjuvanted vaccines have an ingredient that increases the inflammation (to force an immune response to the disease ingredients in the vaccine), and it is believed that that inflammation plays a key role in the development of sarcomas. Adjuvanted vaccines should be avoided for that reason.
The second recommendation can be applied to all injections, and that is that injection sites should be located in an area where if a sarcoma develops, it can easily be removed. That is generally recommended to be the leg (which can be amputated if a sarcoma develops), but I spoke to a vet tech earlier this year who works with an oncologist who recommends the stomach because the chest wall is easy to remove the sarcoma from.
For any injection, I generally weigh the risk of developing an ISS against whatever illness I'm treating. When my cats got a steroid shot for itch relief while the flea medication killed the cheyletiella mites, I requested it in their legs just to be safe. If I was giving fluids to a cat in renal failure, I would not worry about the chance of developing cancer in a few years.
Kevin N. Woodward
ISRN Veterinary ScienceVolume 2011 (2011), Article ID 210982,
Origins of Injection-Site Sarcomas in Cats: The Possible Role of Chronic Inflammation—A Review
Brian Wilcock, Anne Wilcock, and Katherine Bottoms
Can Vet J. Apr 2012; 53(4): 430–434.
Feline postvaccinal sarcoma: 20 years later