The correct insulin injection technique

You can inject insulin in the following areas:

• Abdomen — fastest absorption, avoid 5 cm around the navel;
• Thighs — slower absorption, good for basal insulin;
• Arms — intermediate absorption, but the fat layer is thin and it is hard to inject yourself correctly there, so they are used less often;
• Buttocks — slow absorption, sometimes useful in children [1].

The abdomen is ideal for rapid insulin (the prandial analog, taken with meals) due to faster absorption, while the thighs are good for basal insulin (the long-acting analog), as the figure below shows [2].

Each area has multiple injection sites. Mentally divide the area into squares with 3 cm sides and use a different site for each injection [3]. Do not use areas with lipodystrophy (lumps or indentations), bruises, scars or signs of inflammation. Keep the same area for the same type of insulin (for example, rapid in the belly, basal in the thighs) for the most predictable absorption possible [1].

Systematic rotation prevents lipodystrophy and ensures consistent absorption [4]. Divide the abdomen into four quadrants and use one quadrant per week. Alternatively, move in a circle around the navel, like a clock. Do not go near the navel or the midline! Leave at least two cm between two consecutive injections and, as far as possible, do not reuse the exact same site for the following month [3]. You can make a mental map of the sites used.

Some patients use the left and right sides of the abdomen alternately. For example, Monday, Wednesday and Friday on the left, Tuesday, Thursday and Saturday on the right. For evening basal insulin, keep to the same side for a month and then switch to the other side (in the thigh). Large differences between the left and right sides are worth discussing with your medical team; they may be a sign of lipodystrophy or of a different injection technique. Visually check and periodically palpate the injection areas to detect early any lumps or areas of local fat deposition [5].

Insulin should be injected into the subcutaneous tissue (the fat under the skin), not into muscle or superficially into the skin [6]. With 4 mm needles, you can penetrate perpendicular to the skin, without making a fold. With the usual 6-8 mm needles, either make a skin fold and inject at 90 degrees, or inject at 45 degrees without a fold [7]. Needles longer than 8 mm increase the risk of intramuscular injection and should only be used by people with excess subcutaneous fat.

The 4 mm needle is the first choice for everyone, regardless of age, weight or the thickness of the subcutaneous tissue [8]. The 6 mm needle remains acceptable if you use the technique described above [6]. Accidental intramuscular injection is painful, may bleed slightly, and insulin absorption is up to 50% faster (less pronounced with ultra-rapid analogs) [9]. If you hit the muscle, you will feel greater resistance and pain. Because the insulin reaches the bloodstream faster, the risk of hypoglycemia rises over the following hours. Check your blood glucose more often and keep fast-acting carbohydrate within reach. If your blood glucose drops below 70 mg/dL (3.9 mmol/L), you take 15 g of carbohydrate and recheck after 15 minutes.

Disinfecting the skin with alcohol before insulin injection is not necessary [10]. If the skin is clean, modern recommendations no longer require this for routine situations. Alcohol can dry and irritate the skin with repeated use and does not significantly reduce the (very small) risk of infection [10]. Wash your hands before preparing the insulin and make sure the injection area is visibly clean.

If you prefer to disinfect or your doctor recommends it (weakened immune system, poor hygiene conditions), use 70% alcohol and let it evaporate completely before injection [3]. Wet alcohol can cause a burning sensation when the needle passes through the skin. At home, with needles used only once (in practice, many people reuse them) and normal hygiene, the risk of infection is extremely small.

The official recommendation is to use a new needle for each injection [3]. Needles become slightly dull even after the first use (visible under the microscope), which makes subsequent injections increasingly painful and can traumatize the subcutaneous tissue [11]. Reused needles (especially detached ones) may have crystallized insulin residue and can carry bacteria. In reality, many people reuse needles a few times for economic or convenience reasons [11].

Avoid reusing a needle for more than one day. Never share needles with anyone else, even after disinfection. If you reuse the needle, put the cap back on immediately after use and do not touch the needle with anything. At the first sign of increased pain or resistance to injection, change the needle [12]. Used needles go into a rigid, puncture-proof container, not into the household trash. You can use a sharps box or a thick plastic bottle with a lid. When it is full, hand it in at the pharmacy or at your doctor's office, according to local rules.

Bruises occur when the needle hits and breaks a small capillary vessel [13]. It is normal for this to happen occasionally and it does not affect insulin absorption. Such small bruises may occur somewhat more frequently if you take aspirin (in low dose) or anticoagulants, if you have vitamin K deficiency or increased capillary fragility [13]. Injecting too quickly or using the same needle several times increases the risk of such bruises.

To minimize bruises, inject slowly and steadily, do not massage the area immediately after injection, and use new needles [3]. If blood appears on the skin after insulin administration, apply light pressure (not massage) with a clean pad. Small bruises fade on their own and do not need treatment, although how long that takes varies a lot from one person to another. Show your medical team any bruise that is large, painful, spreading or not clearing up. Avoid the bruised area for subsequent injections until it heals completely.

Use new and short needles (4 mm), inject slowly and steadily and relax the muscles in the injection area [7]. Let a new insulin pen reach room temperature by taking it out of the refrigerator 15-30 minutes before. Avoid areas with many nerve endings, such as the edge of the abdomen, too close to the navel, knees or other joints. Distract yourself and breathe deeply.

If you use insulin from a new refrigerated vial or pen, "cold burn" pain is common [3]. Keep the insulin you are currently using at room temperature for up to three weeks [3]. Some people apply an ice cube for ten seconds before injection to numb the area locally. Data on the effect of anesthetic creams on insulin absorption are limited, so talk with your medical team before using them routinely.

It is not recommended. Although it is not medically recommended, the reality is that many people reuse needles a few times [11]. The assumed risks include:

• Increased pain — the needle becomes visibly dull;
• Lipodystrophy — repeated tissue trauma;
• Local infections — rare but possible;
• Incorrect dosing — insulin crystals can block the needle;
• Cartridge contamination — bacteria can enter through the needle [11].

If you reuse needles for economic reasons, keep in mind that the tip blunts from the very first use, and each following injection hurts more and traumatizes the tissue more. Do not use the same needle for more than 24 hours [12]. Keep the cap on between uses and do not touch the needle. Change the needle immediately if it has bent or you feel increased resistance to injection. Never reuse if you have active infections, a weakened immune system or healing problems in general. Calculate whether the savings made this way are worth the risks. In general, needles are relatively inexpensive.

A little blood after injection is not a problem [13]. It means you hit a small capillary and it is not dangerous at all. Apply light pressure with a cotton pad or clean tissue for 10-30 seconds. Do not massage the area. This can spread insulin in the tissue and accelerate absorption in an unpredictable way.

If bleeding continues for more than a minute or a large bruise appears, apply firmer pressure and possibly ice. Note in your journal if your blood glucose behaves differently after an injection that bled. This is very rare. Faster absorption may occur only if you hit a larger vessel [9]. Frequent bleeding may indicate needles that are too long and hit the muscle, or clotting problems that may require additional investigation (very unlikely) [6].

Unused insulin is stored in the refrigerator at 2-8°C, ideally in the middle section (not in the freezer or on the door where the temperature varies) [3]. The vial or pen in use is kept at room temperature (below 25°C) for up to four weeks, as stated in the package leaflet of most insulins. In practice, three weeks is a safer choice, especially in summer or when the temperature in the house goes above 25°C [14]. Some insulins last longer, others less, so check the package leaflet as well. Do not expose insulin to direct light, extreme heat or freezing [3].

For transport, use an insulated case. Do not leave insulin in the car, especially in summer or winter [14]. At the airport, carry insulin in your hand luggage, accompanied by a medical letter. Insulin should not be shaken violently (it foams and affects dosing). Insulin analogs (long-acting or rapid) no longer need shaking (mixing) before use, as in the case of NPH (neutral protamine Hagedorn) insulin [3]. NPH is an intermediate-acting human insulin, normally cloudy, prescribed less and less often but still available in many countries. Always check the appearance before use. Do not use insulin if it is cloudy (NPH is the exception), if it has crystals or changed color.

You can, but it is not recommended. Technically you can inject through thin clothes (shirt, thin t-shirt), but it is not recommended at all [3]. Textile fibers can contaminate the needle, you cannot see exactly where you are injecting and you may get the correct rotation of injection areas wrong. Thick clothes deflect the needle and affect injection depth.

If you have an emergency and no alternative (hard to imagine), make sure the clothing area is clean. Use a new needle, inject through only a single thin layer of clothing and be very careful about injection depth [15]. Only do this in exceptional cases. Most people find a discreet way to lift their clothing slightly to have direct access to the skin.

Lumps generally appear from repeated injection in the same area [16]. The fatty tissue proliferates abnormally (fat cells multiply and grow larger) in response to insulin's trophic effect, that is, its ability to stimulate tissue growth. The lumps are soft, painless, but absorb insulin unpredictably, sometimes with delay, sometimes not at all [16]. They occur in over half of the people who use insulin, but are much more common in those who do not correctly rotate injection sites or reuse needles [5].

For prevention, systematically rotate injection sites, use new needles, and regularly check and palpate the injection areas [4]. If you already have lumps, completely avoid that area for at least three months. Never inject into lumps: insulin is absorbed unpredictably from them, and blood glucose becomes hard to control [16]. Show your doctor all areas with changes to evaluate your injection technique together. Rarely, lumps may appear through an allergic reaction to the insulin used, in which case changing the type of insulin may be necessary.