How long does Humalog last? In your body and once opened

Humalog starts working in about 15 minutes, is strongest after 1 to 2 hours, and keeps working for about 2 to 5 hours. Once you start using a pen or vial, it lasts 28 days at room temperature. The rest of this article explains where those numbers come from, why your own doses don’t always follow them, and the storage rule that catches people out.

How long does Humalog keep working after you inject it?

Humalog is insulin lispro, a rapid-acting mealtime insulin. Here’s what the American Diabetes Association (ADA) and Diabetes UK say about rapid-acting insulin like it:

	How long after injecting
Starts working	About 15 minutes (ADA)
Strongest effect	About 1 to 2 hours (ADA)
Keeps working	About 2 to 4 hours (ADA), or 3 to 5 hours depending on the brand (Diabetes UK)

The two ranges overlap rather than disagree. Most of a dose’s work happens in the first few hours, but a smaller tail keeps going after that. Humalog’s own prescribing information calls figures like these “only general guidelines”, because people vary.

The label also says Humalog starts working faster and stops sooner than regular human insulin, the older kind of mealtime insulin.

That tail is why a dose from lunch can still be working mid-afternoon. It’s called insulin on board, and it’s what makes doses taken close together overlap. Insulin Pal, for example, counts a Humalog dose for 5 hours, leaning toward the longer end of the range. You can see how the numbers work in full.

Why does Humalog seem to last longer on some days?

The Humalog label says the timing “may vary considerably in different individuals”, and that it depends on the injection site, blood supply, temperature and exercise. In practice, a few things stand out:

• Where you inject. According to the label, Humalog injected into the abdomen reached higher levels in the blood and worked for slightly less time than when injected into the upper arm or thigh.
• Exercise. Moving makes insulin work harder, sometimes for hours afterwards. More in exercise and blood sugar when you take insulin.
• Warmth. The label lists temperature at the injection site among the things that change how a dose behaves.
• Lumpy skin. Injecting into the same spot again and again can cause lumps under the skin, and the label says not to inject there. It also warns that suddenly switching from a lumpy area to healthy skin has been reported to cause low blood sugar, so that change is worth planning with your care team.
• What you ate. Meals high in fat and protein can keep raising blood sugar after Humalog’s strongest hours have passed. See carb counting for beginners.

When do you take Humalog?

The prescribing information says to inject Humalog within 15 minutes before a meal, or right after it. Your care team may give you a different plan, and theirs is the one to follow.

How does Humalog compare with NovoRapid and Fiasp?

The NHS lists Humalog alongside other rapid-acting insulins, including NovoRapid, Admelog, Apidra, Fiasp and Lyumjev. They’re close, but not identical:

• NovoRapid (NovoLog in the US) and Admelog have timings similar to Humalog’s, and Insulin Pal uses the same curve for all three. Admelog is also insulin lispro.
• Fiasp and Lyumjev are faster versions. Diabetes UK describes Fiasp as working even quicker, and Lyumjev is a faster form of the same insulin as Humalog. MedlinePlus notes that Lyumjev is taken at the start of a meal or within 20 minutes after you start eating.
• Humalog U-200, the more concentrated KwikPen, works on the same timeline as standard Humalog, according to the label.
• Humalog Mix 75/25 and 50/50 are premixed insulins with different timings and storage rules. MedlinePlus says an opened Mix pen lasts only 10 days, not 28.

How long does Humalog last once opened?

This is the other thing people mean when they ask how long Humalog lasts. The rules differ a little by country. According to the US prescribing information:

	In the fridge (2–8°C / 36–46°F)	At room temperature (up to 30°C / 86°F)
Unopened pen, cartridge or vial	Until the expiry date	28 days
Opened vial	28 days	28 days
Opened pen or cartridge	Don’t refrigerate	28 days

The rule that catches people out: in the US label, the 28 days at room temperature is a total. It counts time spent unopened and opened together, so a pen that sat in your bag for a week before you started it has about three weeks left.

The UK product information is stricter about unopened insulin: keep it in the fridge until you start using it. Once it’s in use, it gives the same 28 days below 30°C, and adds that a pen shouldn’t be stored with the needle attached.

A few more rules from the labels and MedlinePlus:

• Never use insulin that has been frozen, even after it thaws.
• Humalog should look clear and colourless. Don’t use it if it’s cloudy, coloured, or has particles in it.
• Throw it away if it’s been exposed to extreme heat or cold.
• In a pump, change the insulin in the reservoir at least every 7 days, or sooner if your pump’s manual says so. Change it straight away if it’s been above 37°C (98.6°F).

Talk it through with your care team

If Humalog seems to work much faster or slower than you expect, you get lows several hours after meals, or you notice lumps where you inject, bring it up with your doctor, nurse or diabetes educator. The same goes for switching between Humalog and another insulin: brands that look alike can still work on different timelines.

The short version

• Humalog starts working in about 15 minutes, is strongest after about 1 to 2 hours, and keeps working for about 2 to 5 hours.
• Those are typical figures. Injection site, exercise, warmth and what you eat all change how long a dose lasts for you.
• The tail of a dose is easy to forget, which is why tracking insulin on board helps.
• An opened Humalog pen or vial lasts 28 days at room temperature. In the US, that total also counts any time it spent unopened out of the fridge.
• Never use Humalog that has been frozen, or that looks cloudy or coloured.