Short answer: Low blood sugar (hypoglycemia) is the most common Humalog side effect, as with all insulin. In the label's type 1 trial, flu syndrome affected 34.6%, pharyngitis 33.3% and headache 29.6%, close to regular human insulin. In type 2, headache affected 11.6%. The Patient Information also lists injection site reactions, skin pits or thickening, weight gain, swelling, itching and rash. Humalog has no boxed warning.
What Humalog is approved for: Humalog is a rapid-acting mealtime insulin approved to improve blood sugar control in adults and children with diabetes (section 1).
Source
HUMALOG (insulin lispro) injection, for subcutaneous or intravenous use, prescribing information and Patient Information. Eli Lilly and Company. DailyMed, version published August 10, 2026. DailyMed setid c8ecbd7a-0e22-4fc7-a503-faa58c1b6f3f
No. The current Humalog 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 Humalog.
| Adverse reaction | HUMALOG, % (n=81) | Regular human insulin, % (n=86) |
|---|---|---|
| Flu syndrome | 34.6 | 32.6 |
| Pharyngitis | 33.3 | 33.7 |
| Rhinitis | 24.7 | 29.1 |
| Headache | 29.6 | 22.1 |
| Pain | 19.8 | 16.3 |
| Cough increased | 17.3 | 17.4 |
| Infection | 13.6 | 20.9 |
| Nausea | 6.2 | 15.1 |
| Accidental injury | 8.6 | 11.6 |
| Surgical procedure | 6.2 | 14.0 |
| Fever | 6.2 | 11.6 |
| Abdominal pain | 7.4 | 8.1 |
| Asthenia | 7.4 | 8.1 |
| Bronchitis | 7.4 | 7.0 |
| Diarrhea | 8.6 | 5.8 |
| Dysmenorrhea | 6.2 | 7.0 |
| Myalgia | 7.4 | 5.8 |
| Urinary tract infection | 6.2 | 4.7 |
Common reactions excluding low blood sugar, at 5% or more on insulin lispro or regular human insulin. The comparison is regular human insulin, not placebo.
| Adverse reaction | HUMALOG, % (n=714) | Regular human insulin, % (n=709) |
|---|---|---|
| Headache | 11.6 | 9.3 |
| Pain | 10.8 | 10.0 |
| Infection | 10.1 | 7.6 |
| Pharyngitis | 6.6 | 8.2 |
| Rhinitis | 8.1 | 6.6 |
| Flu syndrome | 6.2 | 8.2 |
| Surgical procedure | 7.4 | 6.8 |
| Measure | HUMALOG U-100 (n=38) | Regular human insulin (n=39) |
|---|---|---|
| Catheter occlusions/month | 0.09 | 0.10 |
| Infusion site reactions | 2.6% (1/38) | 2.6% (1/39) |
A 12-week crossover study of adults with type 1 diabetes using an insulin pump (n=39).
In a 16-week pump study of children with type 1 diabetes, infusion site reactions were similar for insulin lispro and insulin aspart: 21% of 100 patients versus 17% of 198. The most frequent were infusion site redness and infusion site reaction.
In controlled trials, itching (with or without rash) was seen in 30 of 2944 patients on Humalog and 17 of 2969 on regular human insulin. The label gives these as counts, not percentages.
Redness, swelling or itching at the injection site usually clears in a few days to a few weeks, but sometimes means stopping Humalog. Metacresol, an inactive ingredient, has been linked to site reactions and general muscle aches.
Fast improvement in blood sugar control has been linked to a temporary change in eye focus, worsening diabetic eye disease and painful nerve symptoms. Weight gain, swelling from sodium retention and skin pits or thickening (lipodystrophy) can also occur.
Anti-insulin antibodies rose most in people new to insulin, peaked by 12 months and then declined. The label says they did not appear to worsen blood sugar control or require more insulin.
Reported after approval: other insulins given by mistake in place of Humalog, and localized cutaneous amyloidosis (lumps under the skin) at injection sites. Injecting into those lumps has raised blood sugar, and a sudden switch to normal skin has lowered it.
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. Humalog is not used during an episode of low blood sugar, or by people allergic to insulin lispro or any of its inactive ingredients.
Never share a pen, cartridge or syringe. Pens, cartridges and reusable pens are 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 insulins have been reported, so the label is checked before each injection. Humalog U-200 is never drawn from the pen into a syringe: syringe markings will not measure it correctly, which can cause an overdose and severe low blood sugar.
Serious allergic reactions. Severe, life-threatening whole-body allergy, including anaphylaxis, can occur; Humalog 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.
Insulin pump failure. A pump or infusion set malfunction, or degraded insulin, can quickly lead to high blood sugar and ketoacidosis. Pump users are trained to give insulin by injection and keep backup insulin on hand.
From the Patient Information that comes with Humalog:
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.