Previously unknown side effects are often discovered when a new medication gets widespread use. If your injection sites get red and itch, you may be having allergic reactions to the insulin. If you use animal insulin, switching to human insulin will usually alleviate the symptoms. Even if you continue using animal insulin, or have reactions to human insulin, these irritations usually stop one to two months after the symptoms first appear. The person might develop bumps all over the skin.
More serious symptoms are swollen lips, tongue, and trouble breathing.
If such symptoms occur, call an ambulance. Fewer than 1 in 1,000 users will have such a severe reaction. People at higher risk are those who use insulin intermittently and those who have had local skin reactions to insulin in the past. Another allergic-like reaction to insulin is lipoatrophy. Lipoatrophy produces a concave or pitted appearance at the injection site.