Key takeaways:
There are many types of insulin available that work at different speeds to treat diabetes. No matter if they’re rapid-, intermediate-, or long-acting, they all have similar side effects to be aware of.
Common insulin side effects include hypoglycemia (low blood sugar), injection site reactions, and weight gain. Rare side effects of insulin are lipodystrophy (abnormal fat distribution) and swelling in the arms and legs.
It’s important to check your blood sugar (blood glucose) regularly if you use insulin. You should tell a healthcare professional about any concerns that you’re experiencing.
Every lock needs the right key to open it. Without the right key, the lock prevents any movement in and out of the door. This image paints a picture of what happens in diabetes.
Insulin is a hormone that’s made and released by your pancreas. It’s the key that opens up the lock to your cells and lets blood sugar (blood glucose) move inside. This is the way that blood sugar can properly be used as energy. But if you have diabetes, your body may not be able to navigate this process on its own. You may need to administer insulin as a daily medication to act as the key to manage your blood sugar.
There are many types of insulin available for you to choose from. They work at different speeds and are injected at different times throughout the day. Some examples include rapid-acting insulin like lispro (Humalog), intermediate-acting insulin like human insulin (Humulin N), and long-acting insulin like glargine (Lantus).
No matter the type, insulin serves the same purpose overall — to lower blood sugar. They also have similar interactions and side effects, generally speaking.
Insulin side effects at a glance
Common or mild insulin side effects include:
Hypoglycemia (low blood sugar), which may cause shakiness, sweating, and more
Injection site reactions, which may cause redness, swelling, and discomfort where you injected insulin
Weight gain
Headache
Lipodystrophy, or changes in fat distribution
Blurred vision
Mild swelling in the arms or legs
Rare but serious insulin risks include:
Severe hypoglycemia, including confusion, seizures, and loss of consciousness
Hypokalemia (low potassium)
Heart problems
Significant fluid retention
Development of anti-insulin antibodies
Allergic reactions, including anaphylaxis
Many side effects from insulin resolve on their own as your body gets used to the medication. If your side effects don’t improve within a few weeks or if they worsen, discuss them with a healthcare professional. They may be able to adjust the dose or change the type of insulin to better suit your needs.
Assessing your options: Insulin can be categorized based on how long it lasts. Read more about the types of insulin and how long each one lasts.
Tips for success: Experts review the key things you should know before administering insulin — from when to inject to how to store supplies.
Financial side effects: Insulin has a reputation for being pricey, but here’s how you can save — with or without insurance.
Here, we’ll dive deeper into six notable insulin side effects and how to manage them.
1. Low blood sugar
Insulin is intended to lower your blood sugar, but sometimes its effects can be more powerful than you need. Hypoglycemia (low blood sugar) is the most common insulin side effect. This is when your blood sugar drops below 70 mg/dL.
Having blood sugar this low can feel like any of these symptoms:
Shakiness or tingling
Anxiety
Dizziness or lightheadedness
Weakness
Drowsiness
Confusion
Fast heartbeat
Hunger
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If you feel any of these symptoms, it’s important to check your blood sugar right away. If it’s below 70 mg/dL, follow the “15-15 rule.” This means you should eat or drink 15 g of fast-acting carbohydrates. This could include 3 to 4 glucose tablets, ½ cup of fruit juice or regular soda, or 1 tbsp of sugar.
Then, recheck your blood sugar levels in 15 minutes. If it’s still below 70 mg/dL, you can repeat this process. If you still don’t respond to fast-acting carbohydrates by this time or your reading is below 55 mg/dL, it’s time to get emergency medical help.
To prevent hypoglycemia with insulin, it’s important to administer the right dose. Talk to your prescriber about adjusting your units of insulin to keep your blood sugar in the normal range. This range is generally between 80 mg/dL to 130 mg/dL before meals or less than 180 mg/dL 2 hours after a meal. Finding your right dose can take some trial and error and gradual dose adjustments.
2. Injection site reactions
Injecting insulin can lead to pain or discomfort at the site of your injection. It may also swell, itch, or turn red. This is called an injection site reaction. Most of the time these reactions are minor irritations caused by the injection. They’re not considered allergic reactions.
What’s more, injection site reactions are typically not concerning. They’re a temporary problem that should get better on their own. To relieve some of the discomfort, you can apply an ice pack, warm compress, or take an anti-itch medication like diphenhydramine (Benadryl).
To lower the possibility of having this reaction, it’s important to rotate the spot where you inject and bring the insulin to room temperature before injection. Injecting insulin while it’s cold can be more painful.
3. Weight gain
Insulin can cause weight gain. When insulin helps your cells soak up extra sugar from your blood, that sugar is converted into fat. This ultimately helps your body use its sugar supply better, but it can lead to weight gain.
People with diabetes who use insulin can gain an average of 4 lbs (2 kg) in the first year they start insulin. To prevent this type of weight gain, it’s important to pair insulin with a diabetes-friendly diet and regular exercise. These lifestyle changes can also better manage blood sugar and overall diabetes health.
If you see that you’ve gained weight after starting insulin, talk to a healthcare professional for added guidance on lifestyle changes.
4. Headache
It’s possible to develop headaches after starting or changing your insulin dose. While low or high blood sugar levels can cause headaches, a big swing in your blood sugar readings could also be the culprit.
Having a headache may not need quick medical help, but it’s still an important side effect to know about. You can take an over-the-counter medication like ibuprofen (Advil, Motrin) or acetaminophen (Tylenol) to manage the pain. But once you get used to insulin and your blood sugar levels stabilize, headaches will generally improve.
If you keep having headaches or they get worse at any point, there could be another cause. Talk with a healthcare professional about finding the cause and helping treat the headache.
5. Lipodystrophy
Lipodystrophy is when there is a problem with fat distribution in the body. There are two types of lipodystrophy that insulin can cause: lipoatrophy (loss of fat or pits in the skin) or lipohypertrophy (enlargement of the fat or thickened skin). When this happens, it usually shows at the spot where insulin was injected.
It’s important to rotate your injection site to help avoid lipodystrophy. Injecting into areas with abnormal fat distribution can also affect how well blood sugar is managed. That’s another reason why rotating injection sites is key to safe insulin use.
6. Swelling in your arms and legs
Insulin can cause your body to retain sodium and fluid, also known as edema. Holding on to excessive fluid can especially affect your arms and legs.
This is a rare insulin side effect, and it usually goes away on its own. A healthcare professional may recommend lifestyle changes for short-term relief, such as elevating your legs above the level of your heart and limiting salt in your diet.
But if these tips don’t help or if you experience swelling that continues, your prescriber may adjust your dose or suggest starting a diuretic (water pill).
Frequently asked questions
Administering too much insulin can cause your blood sugar to drop too low. Early signs may include shakiness, sweating, or dizziness. You might also feel irritable, confused, or notice a rapid heartbeat. If blood sugar falls too far, it can cause seizures or loss of consciousness — situations that warrant immediate medical attention. For mild symptoms, eating or drinking something with sugar usually helps. But frequent or severe episodes can be dangerous, and emergency care may be needed if symptoms become intense.
Missing a dose of fast-acting (mealtime) insulin can cause your blood sugar to spike soon after eating, especially if your meal is high in carbs or sugar. You may notice symptoms of high blood sugar — such as increased thirst or frequent urination — within a few hours. Skipping a dose of long-acting (basal) insulin usually causes a slower rise in blood sugar, with symptoms like tiredness or thirst developing over the next day or night.
Administering insulin when you don’t need it can cause your blood sugar to drop too low. This can trigger symptoms such as confusion, shakiness, and sweating. If you administer insulin by mistake, check your blood sugar right away and continue checking it regularly for the next several hours. Eating or drinking something with a fast-acting sugar — such as glucose tablets or fruit juice — can help raise your blood sugar when needed. If your readings stay low or your symptoms don’t improve, seek medical help right away.
Administering too much insulin can cause your blood sugar to drop too low. Early signs may include shakiness, sweating, or dizziness. You might also feel irritable, confused, or notice a rapid heartbeat. If blood sugar falls too far, it can cause seizures or loss of consciousness — situations that warrant immediate medical attention. For mild symptoms, eating or drinking something with sugar usually helps. But frequent or severe episodes can be dangerous, and emergency care may be needed if symptoms become intense.
Missing a dose of fast-acting (mealtime) insulin can cause your blood sugar to spike soon after eating, especially if your meal is high in carbs or sugar. You may notice symptoms of high blood sugar — such as increased thirst or frequent urination — within a few hours. Skipping a dose of long-acting (basal) insulin usually causes a slower rise in blood sugar, with symptoms like tiredness or thirst developing over the next day or night.
Administering insulin when you don’t need it can cause your blood sugar to drop too low. This can trigger symptoms such as confusion, shakiness, and sweating. If you administer insulin by mistake, check your blood sugar right away and continue checking it regularly for the next several hours. Eating or drinking something with a fast-acting sugar — such as glucose tablets or fruit juice — can help raise your blood sugar when needed. If your readings stay low or your symptoms don’t improve, seek medical help right away.
The bottom line
Insulin is a lifesaving medication for diabetes, and several types of insulin are available. The most common insulin side effects are hypoglycemia (low blood sugar), injection site reactions, and weight gain. Side effects like lipodystrophy and edema are more rare, but possible. Thankfully, most of insulin’s side effects should get better with time. Talk to a healthcare professional about any concerns you may have with administering insulin.
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References
Akinci, B., et al. (2024). Lipodystrophy syndromes: Presentation and treatment. Endotext.
American Diabetes Association. (n.d.). Hypoglycemia (low blood glucose).
Association of Diabetes Care & Education Specialists. (2020). Pro tips (and tricks) for easier and better insulin injections.
Brutsaert, E. F., et al. (2025). Medication treatment of diabetes mellitus. Merck Manual Consumer Version.
Centers for Disease Control and Prevention. (2022). Treatment of low blood sugar (hypoglycemia).
Church, D. S., et al. (2023). Diagnosis and treatment of anti‐insulin antibody‐mediated labile glycaemia in insulin‐treated diabetes. Diabetic Medicine.
Endocrine Society. (2022). Pancreas hormones.
Sanofi-Aventis. (2025). MERILOG- insulin aspart-szjj injection, solution [package insert]. DailyMed.
