Short answer: Low blood sugar (hypoglycemia) is the most common Humulin R side effect, as with all insulin. The label has no trial table with percentages; it lists reactions from studies and post-approval reports: injection site bruising, pain, redness, lumps and itching, rare severe allergy, low potassium, skin pits or thickening, lumps under the skin, swelling and weight gain. Humulin R has no boxed warning.
What Humulin R U-100 is approved for: Humulin R U-100 is a short-acting human insulin approved to improve blood sugar control in adults and children with diabetes (section 1).
Source
HUMULIN R (insulin human) injection, for subcutaneous or intravenous use, prescribing information and Patient Information. Eli Lilly and Company. DailyMed, version published September 7, 2026. DailyMed setid b519bd83-038c-4ec5-a231-a51ec5cc291f
No. The current Humulin R U-100 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 R U-100.
| Reaction | What the label says |
|---|---|
| Hypoglycemia | The most commonly observed adverse reaction with Humulin R |
| Injection site reactions | Bruising (hematoma), pain, bleeding, redness, nodules, swelling, discoloration, itching, warmth and a lump (mass) at the site |
| Severe allergic reactions | May include anaphylaxis, whole-body skin reactions, rash, angioedema, bronchospasm, low blood pressure and shock |
| Hypokalemia | Insulin shifts potassium into cells, possibly causing low potassium |
| Lipodystrophy | Lipoatrophy (depression in the skin) or lipohypertrophy (enlargement or thickening of tissue) |
| 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 |
| Peripheral edema | Sodium retention and swelling, particularly if poor control is improved by more intensive insulin therapy |
| Weight gain | Attributed to the anabolic effects of insulin |
| Starting or tightening control | Temporary change in eye focus, worsening diabetic eye disease and acute painful nerve symptoms |
| Medication errors | Other insulins given by mistake in place of Humulin R |
| Immunogenicity | Anti-insulin antibodies may form; the rate with Humulin R is unknown |
The label gives no rates and no placebo or comparator column for Humulin R.
| Measure | Value on the label |
|---|---|
| Effect begins | About 30 minutes (range 10 to 75 minutes) |
| Maximum effect | About 3 hours (range 20 minutes to 7 hours) |
| Effect ends | About 8 hours (range 3 to 14 hours) |
| Effect ends with 50 or 100 unit doses in obese subjects | About 18 hours (range about 12-24 hours) |
| Median apparent half-life | About 1.5 hours (range 40 minutes-7 hours) |
Doses in the 0.05 to 0.4 unit/kg range. The label says timing varies considerably between people and within the same person.
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, weight gain, swelling in the hands or feet, itching and rash.
Given into a vein, the effect of Humulin R begins at about 10 to 15 minutes and ends at a median of about 4 hours (range 2 to 6 hours) after doses of 0.1 to 0.2 units/kg.
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 R is not used during an episode of low blood sugar, or by people allergic to Humulin R or any of its inactive ingredients.
Never share needles or syringes. Vial users never share needles or syringes with another person, 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 all insulins. Severe episodes can cause seizures, unconsciousness 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 R 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 R U-100:
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.