A booster is an extra shot of insulin, given in addition to the usual basal or basal/bolus dose, to overcome persistent hyperglycemia or to lower temporarily elevated blood glucose levels. In human diabetes terminology, it is known as either a corrective or correction dose because it's intended to correct the hyperglycemia. It can also be referred to as supplimental insulin. When fast or rapid-acting insulin is given regularly to manage the rise in blood glucose from meals, it is known as a bolus dose.
Booster shots in cats and dogs are not widespread practice, not recommended for beginners, and some find them too risky even for advanced use.
Those who do use them tend to prefer to use a short-acting insulin to keep effects well-bounded and understood. It is possible to use longer-acting insulins as boosters, but it's harder to predict their effects and interactions with the basal dose and so is usually not recommended. Short-acting insulins can easily cause hypoglycemia and so must be used only in limited doses and with plenty of experience.
- When exercise, infection, or medication temporarily raises blood glucose levels. (Raising the basal dose is sometimes too risky, but a short-term booster is more predictable and safer in some cases).
Illness places the body under stress, which can raise blood glucose levels. It also goes to work trying to rid itself of the problem; this causes some hormonal release of some of the same counter-regulatory hormones the body releases when it feels it's being threatened with hypoglycemia. This can make it difficult to keep in good control using the same amount of insulin when there's no illness. Humans with diabetes have "sick day" plans; on those days, more insulin, more blood glucose testing and checking for ketones may be needed.
- When readjusting dosage after regulation is lost due to infection, weakened/changed insulin, or other reasons. It's safest when changing basal dose to drop the dose below the anticipated correct number, then work up very slowly. But during that readjustment time, small booster doses can keep the animal from hyperglycemia and the complications it causes.
- When the animal's current insulin dose gives good response and a low peak BG level, but not enough duration. In this case, until the insulin has "settled" and achieves greater duration or the insulin is changed, a small booster dose between shots can keep blood glucose levels in a safer range.
- Boosters can also, with enough experience, be used to help diagnose the reasons behind persistent hyperglycemia after changing insulin. See this thread on FDMB.