Insulin is used in the treatment of patients with most types of diabetes. Need for insulin depends upon the degree of insulin deficiency.
All Type 1 diabetics need insulin treatment; individuals dealing with Type 2 diabetes will require insulin over time as their beta-cell function will gradually decline.
Some of the insulins used are either human insulin or insulin analogues, in the past Animal-sourced insulins were the only insulins available and a lot of people had allergic reactions to them, they are no longer produced now.
Still, they are made especially for people with a reaction to biosynthetic insulin which is ironical.
Some of the common side effects are mild allergic reactions such as swelling, itching or redness around the injection site this can happen for a short time and is usually self-limiting

If you do notice these reactions every time you take an injection, then it is time to bring it to notice to your care provider.
Some of the other serious side effects of insulin is Hypoglycemia or low sugars, and this can happen when you are :- Taking a higher dose of insulin
- Delay in your food timings
- Inadequate food intake
- Overexertion and exercise.