Insulin Clinics / Starting and managing

Insulin side effects

Most side effects of insulin are predictable and manageable. Low blood sugar matters most; others, such as weight gain or temporary blurry vision, are common and usually settle.

Last updated: October 2026 · Based on ADA Standards of Care, FDA prescribing information, and NIDDK · General information, not medical advice — see our editorial policy

Low blood sugar (hypoglycemia)

The most common and most important side effect of insulin is low blood sugar — glucose below 70 mg/dL (3.9 mmol/L). It happens when there is more insulin working than the body needs at that moment: a missed or smaller meal, more activity than usual, alcohol, or a dose that is too high.

Early symptoms include shakiness, sweating, a fast heartbeat, hunger, anxiety, and tingling lips. Later symptoms include confusion, slurred speech, unusual behavior, and drowsiness. Treat with 15 grams of fast-acting carbohydrate and recheck in 15 minutes. The low blood sugar page has the full steps, including when someone else needs to give glucagon.

Weight gain

Many people gain some weight after starting insulin, most often in the first months. Part of this is the body regaining calories it was losing in the urine when glucose was high; part is insulin's normal role in storing energy. Frequent lows that are treated with extra food can add to it.

The amount varies a lot between people. If weight is a concern, raise it with your care team: adjusting the regimen, reviewing low-blood-sugar episodes, and — in type 2 diabetes — adding or changing other medicines can all help. Do not cut insulin doses on your own to avoid weight gain; that can be dangerous, particularly in type 1 diabetes.

Injection site reactions

• Redness, small bruises, or a little bleeding at the site are common and usually harmless.
• Itching or a raised bump that fades within days can happen, especially in the first weeks.
• Lipohypertrophy — soft, rubbery lumps from injecting into the same spot repeatedly. They do not hurt, which is why people keep using them, but insulin absorbs unpredictably there. See injection technique.
• Lipoatrophy — dents in the skin from loss of fat. This was more common with older, less purified insulins and is now rare; mention it to your team if you notice it.
• Pump and CGM sites can also become irritated or infected. Redness that spreads, warmth, pain, or pus needs attention.

Fluid retention (insulin edema)

Some people notice swelling of the ankles or feet, or a quick gain of a few pounds of water weight, soon after starting or intensifying insulin — particularly when glucose had been very high for a long time. It is uncommon and usually settles on its own over a few weeks. The risk of fluid retention and heart failure is higher when insulin is combined with thiazolidinedione medicines (such as pioglitazone). Tell your care team about any new swelling, and seek prompt care for shortness of breath, especially when lying flat.

Changes in vision

When blood sugar falls quickly after being high for a long time, fluid shifts in the lens of the eye can make vision blurry for a few weeks. This usually settles as glucose stabilizes, so it is often best not to get new glasses during this time.

Separately, people with existing diabetic eye disease (retinopathy) can occasionally see it worsen temporarily when glucose is brought down rapidly. This is one reason an up-to-date eye exam is recommended around the time insulin is started or intensified. Report sudden vision loss, new floaters, or flashes of light urgently.

Less common effects

• Low potassium (hypokalemia). Insulin moves potassium into cells. This matters mostly in hospital, during treatment of DKA, or with certain other medicines, and is checked with blood tests.
• Treatment-induced neuropathy. Rarely, a rapid drop in A1C after long-standing high glucose is followed by new nerve pain. It is uncommon and usually improves over time, but it is worth reporting.
• Allergic reactions. True allergy to insulin is rare. Generalized itching, hives, or a widespread rash should be reported. Swelling of the face, lips, or throat, or difficulty breathing, is an emergency.
• Inhaled insulin has its own side effects, mainly cough and a small reduction in lung function — see inhaled insulin.

Things that are often blamed on insulin

Insulin does not cause diabetes complications such as kidney, eye, or nerve damage — high glucose does, and insulin lowers it. When someone seems to get worse after starting insulin, it usually reflects how long glucose was high before insulin was started. Feeling tired or "off" when glucose first comes down is also common, and usually passes as the body adjusts to normal readings.

When to contact your care team

• More than one or two lows a week, any low below 54 mg/dL (3.0 mmol/L), or any low you needed help to treat.
• New swelling, rapid weight gain, or vision changes.
• Lumps, dents, or persistent redness at injection sites.
• A rash, hives, or itching beyond the injection site.

Frequently asked questions