Bruises and swellings on the stomach (or more accurately the abdominal wall) due to insulin are caused by two distinct mechanisms and are not usually variations of the same thing. However, occasionally a bruise will be severe enough so as to cause an actual swelling.
Bruising is due to leakage of blood from small vessels penetrated at the time of the insulin injection.
Some people are more likely to bruise than others, including the elderly and those on anticoagulant or antiplatelet medications (blood thinners) such as warfarin, heparin, clopidogrel, ticlopidine, and others.
Some people on routine doses of aspirin for protection from stroke and heart attack will note an increased tendency to bruise with their insulin injections, while others do not. Injection tools or technique may also play a role.
Those with relatively small amounts of body fat on their abdominal walls (or thighs or arms if you use them) need to be sure to lift and gently pinch the skin and insert the needle at an angle in the middle of the pinched tissue. Injecting the needle directly and to the hilt may lead to an inadvertent injection into the muscle.
This will change the pattern of action of the insulin and lead to a bruise or sometimes a painful bleed into the muscle itself. Some types of needle are more prone to cause bruising in some individuals than others. If you bruise easily, use the smallest diameter (gauge) of needle you can find. Remember that the higher number of gauge means a smaller diameter.
Swellings, especially if they are chronic, are often due to a property of the insulin itself. Insulin is one of the hormones responsible for growth and maintenance of our tissues including muscle protein and fat stores.
If insulin is injected repeatedly in the same area, additional fat can become laid down in that spot. This takes on the appearance of a painless fatty mound (lipohypertrophy), which can be somewhat disfiguring. Fortunately, these fatty mounds will disperse if the insulin is no longer injected there, although it may take several weeks or months to remodel. To avoid these, the insulin injection sites should be rotated.
Keeping a simple map of your abdominal wall, divided into a grid of 20 or 30 boxes and checking off a box after you have used it, will allow you to avoid using the same spot repeatedly within a 3- or 4-week period. Rarely, the opposite of lipohypertrophy may occur, i.e., loss of body fat in a localized or more generalized way.
This is known as lipoatrophy. This is sometimes pre-ceded by a viral infection, but more often the underlying cause cannot be identified. Red or itchy areas at the site of injection may indicate an allergic reaction to the insulin or a component of it.
This is quite rare with modern insulins, but will necessitate a process known as desensitization, which needs to be performed under the supervision of a specialist.
Larger red or discharging swellings, which are shiny and tender, indicate an abscess due to infection and you should seek medical attention promptly.