Short answer: Low blood sugar (hypoglycemia) is the most common Humulin N side effect, as with all insulin. The label has no trial table with percentages; it lists reactions reported after approval: redness, swelling and itching at the injection site, rare whole-body allergy, swelling from sodium retention, skin pits or thickening, lumps under the skin, weight gain and insulin antibodies. Humulin N has no boxed warning.
What Humulin N is approved for: Humulin N is an intermediate-acting human insulin approved to improve blood sugar control in adults and children with diabetes (section 1).
Source
HUMULIN N (insulin isophane human) injectable suspension, for subcutaneous use, prescribing information, Patient Information and Instructions for Use. Eli Lilly and Company. DailyMed, version published December 31, 2025. DailyMed setid f6edd793-440b-40c2-96b5-c16133b7a921
No. The current Humulin N label has no boxed warning.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Humulin N.
| Reaction | What the label says |
|---|---|
| Hypoglycemia | The most common adverse reaction of insulins, including Humulin N (section 5.3) |
| Allergic reactions | Redness, local swelling and itching at the injection site, usually self-limiting; severe cases of whole-body allergy (anaphylaxis) have been reported |
| Peripheral edema | Sodium retention and swelling, particularly if poor blood sugar control is improved by more intensive insulin therapy |
| Lipodystrophy | Lipoatrophy (depression in the skin) or lipohypertrophy (thickening of tissue) at injection sites |
| Localized cutaneous amyloidosis | Lumps at the injection site; injecting into them has raised blood sugar, and a sudden switch to an unaffected site has lowered it |
| Weight gain | Attributed to the anabolic effects of insulin and less sugar lost in the urine |
| Immunogenicity | Antibodies that react with human insulin have been seen with all insulins, including Humulin N |
The label gives no rates and no placebo or comparator column for Humulin N.
| Measure | Value on the label |
|---|---|
| Median maximum glucose-lowering effect (n=16) | 6.5 hours (range 2.8 to 13 hours) |
| Median peak insulin level in the blood | About 4 hours (range 1 to 12 hours) |
| Mean apparent half-life | About 4.4 hours (range printed as 1-84 hours) |
The label says these timings vary between people and within the same person, and are general guidelines only.
The Patient Information lists the most common side effects as low blood sugar, allergic reactions including injection site reactions, skin thickening or pits at the injection site, itching, rash, weight gain, and swelling of the hands and feet.
Humulin N is an intermediate-acting insulin with a slower onset and longer duration than regular human insulin. How fast it is absorbed depends on the injection site, physical activity and other factors.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Who should not use it. Humulin N is not used during an episode of low blood sugar, or by people who have had an allergic reaction to Humulin N or its ingredients.
Never share a pen or syringe. A KwikPen is never shared, even if the needle is changed, and vial users never share needles or syringes, because of the risk of passing on blood-borne infections.
Changes in insulin regimen. Changing insulin strength, maker, type, injection site or method can cause high or low blood sugar, so changes are made under close medical supervision with more glucose checks. In type 2 diabetes, other diabetes medicines may need adjusting.
Low blood sugar. Hypoglycemia is the most common reaction to insulin. Severe episodes can cause seizures or death and can affect driving. Awareness of lows may be reduced with long diabetes, nerve damage, beta-blockers or repeated lows. Kidney or liver problems can raise the risk.
Medication mix-ups. Mix-ups between insulin products have been reported, so the insulin label is checked before each injection.
Serious allergic reactions. Severe, life-threatening whole-body allergy, including anaphylaxis, can occur; Humulin N is stopped if it does.
Low potassium. All insulins shift potassium into cells and can cause low potassium, which untreated can cause breathing paralysis, heart rhythm problems and death.
Heart failure with TZDs. Thiazolidinediones (such as pioglitazone) with insulin can cause fluid retention that may lead to or worsen heart failure.
From the Patient Information that comes with Humulin N:
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.