Sometimes I wonder why people come to a vet in the first place.
Earlier this week my colleague received a call from a vet who was boarding a patient of ours. The cat had recently been diagnosed as a diabetic and had just started on insulin injections. The vet was calling because the original prescription had been written for 2 units, and someone had crossed that out and written 1 unit. It took a little digging, but apparently the owner had done this. Why? She had switched from dry to canned food and for whatever reason decided that this meant she needed to decrease the insulin dosage. The cat wasn't regulated yet and she had never called to check with one of us.
It's not an isolated incident. Several months ago I talked to a client who hadn't been consistent with the insulin doses on their dog and on a routine exam the blood glucose was too high. Apparently the owner was changing the dog's dosage every day based on it's mood. Nope, they weren't checking the blood sugar levels at home or talking to us about the case. They were just altering insulin from dose to dose based on how the dog was acting.
Insulin is a serious drug with serious consequences. If you give too little the blood sugar remains high, which could lead to life-threatening situations (ketoacidosis) if there are prolonged elevations. At the minimum the pet is uncomfortable and can have chronic changes such as cataracts. Give too much and the blood sugar can drop precipitously, reaching the point where the brain is starved of energy. A very low glucose level can result in neurological signs, seizures, and even a coma. There are also several different types of insulin! Some act longer than others, so you can't just switch from one to another while keeping the dosage the same. If you go from a short-acting insulin to a long-acting one you may have to decrease the dose or frequency in order to prevent hypoglycemia.
Altering insulin doses should never be done without veterinary advice and counseling. We make changes based on clinical symptoms at home as well as repeated blood tests to measure the glucose levels. When you are nearing proper regulation there won't be significant behavioral or personality changes, so you can't tell just by watching a pet. For numerous reasons an owner should never change or stop insulin that their vet has prescribed without first talking to the vet. Ignoring this advice can lead to very serious health problems.