A diabetic usually injects insulin prior to a meal because the substance takes between 10 and 45 minutes to start working. Injecting it after a meal means blood sugar spikes before the insulin has time to counteract it. However, there are some circumstances in which it makes sense to inject insulin after a meal.
Diabetic Children Who Are Picky Eaters
Insulin dosages at mealtimes are partially based on the number of grams of carbohydrates that are going to be consumed. If a diabetic child refuses to eat all of her dinner but has already received an insulin dose that was based on her consuming her entire meal, a dangerous hypoglycemic (low blood sugar) reaction can occur. Consequently, some parents prefer to administer an injection immediately after a meal so they can be sure the dosage is correct.
Low Blood Sugar
If blood sugar levels are low before mealtime, a diabetic may choose to inject right after eating a small meal to give levels time to return to normal.
Foods with no carbohydrates, such as cheeses and meats, don't require insulin to balance them, so if a diabetic injects after eating a meal that contained no carbohydrates, it may be because she was simply snacking and it is time for their normal scheduled injection.
On occasion, even the most careful diabetic will forget an injection and have to administer it after eating.
Injecting insulin after eating without a specific medical reason is a dangerous long-term practice. In order to avoid serious complications, diabetics should strive to keep their blood sugar level as even as possible; the constant post-meal spikes that will occur if they inject insulin after (or while) eating can have lethal consequences.