NPH insulin, an intermediate-acting insulin
Isophane insulin
An intermediate-acting (NPH) insulin used for background cover, with a noticeable peak in its action.
What is Isophane insulin?
Isophane insulin, often called NPH, is an intermediate-acting insulin used to provide background cover, usually injected once or twice a day. Unlike the newer flat basal insulins, it has a noticeable peak in its action a few hours after injection, which can raise the risk of a low at certain times, including overnight. It is a cloudy insulin that must be gently mixed before use. Its main risk, like all insulin, is a hypo, so it is important to recognise and treat the warning signs. Always rotate injection sites and never share your pen or needles.
Education and reference only. This is a plain-language guide to Isophane insulin — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Isophane insulin (NPH) is an intermediate-acting human insulin used as the background, or basal, part of insulin treatment in type 1 and type 2 diabetes. It is made by adding a substance that slows the insulin's release, so it lasts longer than fast-acting insulin but has a clear peak a few hours after injection and does not last a full day for most people. Because of this it is often taken twice a day, and it is also a component of many premixed insulins. It is a cloudy suspension that must be gently mixed before each injection, and it comes in pens, cartridges and vials.
How it works
Insulin lets glucose move from the blood into the cells and stops the liver releasing extra glucose. In diabetes too little insulin is available or it works poorly, so glucose rises. Isophane provides longer-lasting background insulin than a fast-acting one, but it works in a wave: it starts to act after a delay, peaks a few hours later, then tails off. This peak can be useful but also means a higher chance of a low at certain times, including overnight, which is why timing, food and monitoring matter.
Company & origin
Originated / developed by: Novo Nordisk (Insulatard); Eli Lilly (Humulin I); Sanofi (Insuman Basal).
A long-established intermediate-acting human insulin, often known as NPH, used in the UK as a background insulin and as a component of premixed insulins.
What it treats
Conditions Isophane insulin is used for
Practical use
How to take Isophane insulin
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Gently roll and tip the pen or vial to mix the cloudy insulin evenly before each injection; do not shake hard.
- Inject under the skin once or twice a day as advised, at roughly the same times, into the tummy, thigh, buttock or upper arm.
- Rotate your injection site within an area each time to prevent lumpy or hardened skin that changes absorption.
- Use a new needle each time and never share a pen or needle with anyone else.
- Because it peaks a few hours after injection, be alert to lows at that time and overnight, and carry fast-acting sugar.
- On sick days keep taking your insulin, check your blood glucose more often and follow your sick-day plan.
Weighing it up
Advantages & disadvantages of Isophane insulin
Advantages
- A long-established, well-understood and usually lower-cost background insulin.
- Its peak can help cover glucose at particular times of day when timed well.
- Also forms the background part of many convenient premixed insulins.
Disadvantages
- Its peak and shorter duration can raise the risk of lows, including overnight, compared with flat basal insulins.
- It is cloudy and must be mixed gently before each dose, and inconsistent mixing gives an unpredictable effect.
- It often needs twice-daily injection and careful timing around meals.
Practical use
Good to know
Isophane is an intermediate-acting background insulin with a distinct peak, which is the main thing that sets it apart from the newer flat basal insulins. Because of that peak, the timing of injections and meals matters, and there can be a greater risk of a low several hours later or overnight. It is a cloudy insulin, so the pen or vial must be gently rolled and tipped to mix it evenly before each injection; using it without mixing can give an unpredictable effect. The key safety point is hypoglycaemia: warning signs include shakiness, sweating, hunger, a fast heartbeat and confusion, treated with fast-acting sugar then a snack. Inject under the skin, rotate sites to avoid lumpy skin, and never share a pen or needle. On sick days keep taking your insulin and follow your sick-day plan.
Who should not take it / use with caution
- It must not be used during an episode of low blood glucose (hypoglycaemia).
- People with a known allergy to isophane insulin or its ingredients should not use it.
- It is not suitable on its own for diabetic ketoacidosis, which needs urgent hospital treatment with fast-acting insulin.
Monitoring
- Regular blood glucose checks, including around the time the insulin peaks and overnight, plus HbA1c over time.
- Reviewing injection sites for lumps or hardened skin that can affect absorption.
- Watching the frequency and pattern of hypos to adjust timing and treatment.
Side effects
- Hypoglycaemia (low blood glucose) is the most important side effect, often related to its peak and possible overnight.
- Redness, itching or lumpy, hardened skin at injection sites if sites are not rotated.
- Weight gain and, rarely, allergic reactions.
Key interactions
- Other glucose-lowering medicines, alcohol and some painkillers can increase the risk of a hypo.
- Steroids, some diuretics and certain other medicines can raise blood glucose and reduce insulin's effect.
- Beta-blockers can mask the warning signs of a hypo, so extra glucose monitoring may be needed.
Available as: Cloudy suspension for injection under the skin, in prefilled pens, cartridges and vials.
Answers
Isophane insulin: frequently asked questions
What does intermediate-acting mean?
Isophane (NPH) lasts longer than fast-acting insulin but not a full day, and it peaks a few hours after injection, so it sits between the rapid and the long-acting insulins.
Why is my insulin cloudy?
Isophane is a cloudy suspension by design; you must gently roll and tip it to mix it evenly before each injection so it works predictably.
When am I most likely to have a low?
Because it peaks a few hours after injection, lows can occur around that time and overnight, so monitor then and carry fast-acting sugar.
Can I share my insulin pen?
No. Pens and needles must never be shared, even with a new needle, because of the risk of passing on infection.
Should I keep taking it when unwell?
Yes. Illness often raises blood glucose, so keep taking your insulin, monitor more often and follow your sick-day advice.
The wider class
About Insulin
Isophane insulin belongs to the insulin class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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