Insulin is required for all animal life (excluding certain insects). Its mechanism of action is almost identical in nematode worms (e.g.C. elegans), fish, and mammals, and it is a protein that has been highly conserved across evolutionary time. Insulin must be administered to patients who experience such a deprivation. Clinically, this condition is called diabetes mellitus type 1 with or without the incuring side effects
Insulin is a protein hormone that is used as a medication to treat high blood glucose. This includes in diabetes mellitus type 1, diabetes mellitus type 2, gestational diabetes, and complications of diabetes such as diabetic ketoacidosis and hyperosmolar hyperglycemic states. It is also used along with glucose to treat high blood potassium levels. Typically it is given by injection under the skin, but some forms may also be used by injection into a vein or muscle.

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The common side effect is low blood sugar. Other side effects may include pain or skin changes at the sites of injection, low blood potassium, and allergic reactions. Use during pregnancy is relatively safe for the baby. Insulin can be made from the pancreas of pigs or cows. Human versions can be made either by modifying pig versions or recombinant technology.[5] It comes in three main types short–acting (such as regular insulin), intermediate–acting (such as NPH insulin), and longer-acting (such as insulin glargine)
If too much insulin is delivered or the person eats less than he or she dosed for, there may be hypoglycemia. On the other hand, if too little insulin is delivered, there will be hyperglycemia. Both can be life-threatening.
Diabetes is difficult to cure. This is a condition in which the blood sugar levels increase and the production of insulin decreases. Many diabetic patients with type I or II form takes insulin injections. A diabetic patient injects insulin because the insulin levels falls due to its slow production. Therefore, few patients have to regularly inject insulin to stabilise blood sugar levels in the body and to stay healthy. Insulin injections have few common and rare side effects on the body. Check out how these injections can have an effect on a diabetic patient.
 Common side effect of insulin injections: 

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Low blood sugar levels. Once you start depending on the injections, your blood sugar levels will lower every time. 
* As the blood sugar decreases, sweating, nausea or rapid breathing can also be seen.
 * If the blood sugar level lowers substantially then the patient can even faint.
 * Skin irritation or inflammation from the injection needle. Few diabetic patients have to inject on a regular basis and this can affect the skin tissues. 
* When the blood sugar levels decrease, the body metabolism also lowers. This also leads to metabolic changes in the brain often leading to seizures. It is a rare condition that needs immediate attention from the doctor. 
Dizziness due to the lower glucose levels. The brain functions slowly and this can make you feel more toxicated. 
* Hypoglycemia is another side effect of taking insulin injections. Too much of insulin in the body lowers blood pressure. This leads to headache, drowsiness, weakness and rapid heart rate.
 * In extreme cases, the side effects of insulin injections can lead to hyperglycemia. The symptoms of this condition are extreme thirst, frequent urination and laziness. 
* For few diabetic patients, insulin injections can lead to skin allergies such as swelling and itching.
In some cases Side effect of insulin scare patients away from treating their selves at some point.

Source: Healthskit