Everything You Ever Wanted to Know About Injecting Insulin…

But Didn’t Know to Ask

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Everything You Ever Wanted to Know About Insulin Injections

Just take your shot. What could be easier, right? Well, you’d be surprised how many errors are made by “veteran” insulin users. It turns out there’s nothing basic about the basics of insulin injections. However, you can improve your technique. This article takes a look at the nitty-gritty details behind successful insulin delivery, why they matter, and how to avoid common pitfalls.

The gear

Realistically, there are two delivery systems when it comes to injecting insulin: syringes and pens. Yes, there are pumps, but that’s a whole other subject. And yes, there are jet injectors, but they are not widely used.


The first-ever human insulin shot was delivered by syringe in 1922, and here in the United States, more than half of all insulin is still delivered via syringe.

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Syringes used to be made of glass, had to be sterilized between uses, and had long, thick, steel surgical needles that could be resharpened on a kitchen whetstone. (No kidding.) But syringes have come a long way since then. Syringes are now disposable, the barrels are made of plastic, and the needles are thin, high-tech, multi-beveled, and coated with lubricants to make them enter the skin smoothly. (Bevels are the slanted surfaces on a needle that create a sharp point.) In the old days, the needle and the syringe were separate components. Nowadays most insulin syringes come with the needle attached.

People who use syringes almost always purchase insulin in vials. Vials are glass bottles that generally hold 1,000 units of insulin.


Insulin pens date from the mid-1980s, and while syringes still predominate in the United States, much of the rest of the world has traded in syringes for insulin pens. Pens currently come in two varieties: disposable, prefilled pens, and reusable pens that take a prefilled cartridge of insulin. Pens have a dial on the base of the pen that’s used to select the size of the dose: You simply “dial up” your dose.

Reusable pens are made of metal, are relatively heavy, and are designed to hold prefilled, 300-unit insulin cartridges. Some reusable pens deliver insulin doses only in full units, and some can deliver half-units of insulin.

Disposable pens are thinner, lighter, and made of plastic. They come prefilled with generally somewhere from 300 units to 600 units of insulin. When they’re empty you throw the whole pen away and get out another. Disposable pens are available for almost every type of modern insulin — basal, rapid-acting, and mixtures — but they can only deliver insulin in full units.

Both types of pens are designed to be used with a disposable pen needle, a short, thin needle attached to a base that screws or snaps onto the pen. Except for pharmaceutical sample packs, pens don’t come packaged with needles, so if you’ve been prescribed pens, make sure you have a prescription for pen needles too.

Preparing your dose

Drawing up a dose of insulin into a syringe involves multiple steps, so it’s not surprising that many people forget some of them over time (or perhaps never learned them in the first place). Dialing up a dose with a pen is much simpler, but it’s still possible to make mistakes.


The procedure for drawing up a dose of insulin using a syringe is as follows: Uncap the needle. Pull the plunger out to fill the syringe with air. Hold the insulin vial upside down. Insert the needle into the vial. Inject the air from the syringe into the vial by pushing in the plunger. Then pull the plunger back down to fill the syringe with insulin. Tap the barrel of the syringe to move any bubbles to the top, and push the plunger to eject them from the barrel. Pull the syringe out of the vial and push out any extra insulin, if necessary. You are now ready to give an injection.

Why the whole ritual of drawing air into the syringe and shooting it into the vial? Basic physics. An insulin vial is a sealed environment. If you try to draw insulin out without first injecting air in, you’re fighting a vacuum. (Of course, it’s not a perfect vacuum. The vial’s flexible plastic seal is designed to allow multiple needle stabs while keeping the vial from springing a leak, and air can eventually work its way in. But still, if you don’t inject some air into the vial first, filling your syringe takes a lot longer.)

So how much air do you inject? About the same amount as the dose you need to withdraw. The air will displace the insulin in the vial. When you shoot the air into the vial, you raise the pressure inside it, making it easier to draw out the contents. In fact, if you simply release the plunger after pushing in the air, the pressure inside the vial will push the plunger back out again, and the syringe will largely self-fill. You might want to inject about 5 percent to 10 percent more air than your dose, because you actually want the first fill of the syringe to be more than the dose you require. This lets you tap any air bubbles that form to the top of the syringe and eject them and any excess insulin back into the vial before you remove the syringe. This way, you won’t need to “top off the tank” a second time if bubble removal leaves you a few units short.

Speaking of bubbles, why worry about air bubbles, anyway? It’s not for the reason most people think. An air bubble in an insulin syringe poses no direct health threat. If you inject air into your body along with your insulin, it won’t kill you because you are injecting the insulin into the fat layer under the skin, not directly into a vein. (It’s true that a large amount of air injected into a vein could be dangerous.) The problem with bubbles in a syringe is that they displace insulin, and if you have a large number of bubbles, you aren’t getting the amount of insulin you think you’re getting. Simply put: Air bubbles cause under-dosing.

Insulin pens

To prepare your dose when using an insulin pen, uncap the pen, attach a pen needle, then dial up your dose using the dial on the end of the pen opposite the needle. Now you’re ready to go.

If you’re using a new disposable pen or a new cartridge in a reusable pen, you need to “prime” the pen before your first dose by dialing in a very small dose and expelling it into the air. This ensures that the plunger is fully advanced and ready to push out insulin.

Properly used, pens don’t generally develop bubbles, so there should be no need to tap them out before injecting. However, leaving a pen needle on the pen between uses can lead to bubbles.

Incidentally, if for some reason your pen malfunctions and you need to withdraw the insulin from the reservoir with a syringe, there’s no need to inject air into it first. Pen reservoirs aren’t sealed environments like insulin vials, so you can just “suck” the insulin out of the pen with a syringe.

Where to inject

You can inject anywhere on your body where there’s fat, which is pretty much everywhere except your forehead. Popular (and perfectly acceptable) injection sites include the abdomen, upper leg, and upper arm. Or you can always inject into your butt — especially what’s formally called the upper buttocks. What’s wrong with the lower buttocks? Not much other than that you sit on them, which could add extra pressure that might affect absorption rates or irritate the injection site.

Generally speaking, the extremities — lower arms and lower legs — aren’t used because they’re too far from your center of mass. It would take a long time for injected insulin to circulate from your ankle to the rest of your body. You should also avoid areas of scar tissue — including your bellybutton — because they don’t absorb insulin well. If you inject into your abdomen, steer clear of your bellybutton by two inches.

All of that said, different parts of your body absorb insulin at different rates, so for predictable results you should choose one general area and stick with it. However, it’s also important not to inject too many times into the same place of your chosen body area. The risk is that repeated stabbing of a needle into the same site can lead to scarring under the skin, which leads to erratic insulin absorption. While this was a bigger issue in the days of larger needles, it’s still possible to injure tissue with today’s small needles.

To escape this risk you should rotate — or routinely change — your injection sites. This means that for each shot, you should avoid the spots you injected into recently. The Joslin Diabetes Center recommends keeping each injection an inch apart and not reusing a site for two weeks. For some people, this is a challenge. A person with Type 1 diabetes on multiple daily injection therapy could take eight or more shots in a day, and it’s easy to forget where you last injected. It also might seem like you would run out of skin landscape quickly, but in fact, for most people this is not true.

There are various rotation methods, including small temporary tattoos to mark your sites, but establishing a personal rotation pattern, and then sticking to it, is probably the best way to ensure you aren’t overloading your “favorite” injection sites. For example, if you use your abdomen, you can start near the top, right under the rib cage, near the center line of your body. The next shot can be one inch to the left, and so on until you reach your extreme left side. Then drop down an inch and work your way back. Think of your stomach as a chess board, and each shot is one square over. When you have placed a shot in each square, move to the other side of your stomach and do the same thing. Depending on how big a person you are, each side of your stomach could easily hold 36–72 injection sites (this assumes 6–12 injection sites left to right, in six rows top to bottom between your ribcage and pelvis).

How to inject

Both syringes and pens should be held perpendicular to the skin, at a 90-degree angle. Place the tip of the needle on the skin and gently press it in. There’s no need to use excessive force — modern needles are super-sharp, beveled, and coated with lubricants. They’ll slide through your skin with minimal pressure, so don’t “ram it home.” There’s also no need to press the syringe or pen itself hard against your skin. If you commonly have bruises at your injection sites, you’re likely pressing the device too forcefully against your skin. It’s only necessary to get the needle in.


To push in the plunger of a syringe, you can use either your index finger or your thumb. As soon as the plunger is fully depressed, the medicine is in, and you can pull the needle back out.

Insulin pens

For pens, most people find it most comfortable to grip the base of the pen with a loose fist and to rest their thumb on the plunger. Once you’ve inserted the needle and depressed the plunger, hold the pen in place for no less than 10 seconds.

Holding the pen in place for at least 10 seconds is vital, because the injection process with a pen is much slower than you’d expect. A pen is a hydraulic delivery system. It uses a plunger pushing on a column of fluid to deliver your medicine. When you depress the plunger of a pen to its base, the delivery has only just begun.

If you want to see this process in action for yourself, take a pen and dial up 10 or 15 units. Hold the pen over a sink and press the plunger to deliver. Watch the flow of insulin and see how long it continues to flow after the plunger is fully depressed. Pen users who pull the pen out as soon as the plunger has hit bottom are guaranteed to under-dose themselves. The bottom line: Don’t pull out prematurely.

Both syringes and pens

After you’ve injected your insulin with either type of device, don’t massage the injection site. Doing so can interfere with the absorption of the insulin, creating an unpredictable glucose response. Likewise, avoid injecting into your arm if you are about to lift weights or into your leg if you are about to go for a run; the “exercised” area will absorb insulin more quickly than you’d normally experience.

Correct needle use

Modern syringes and pen needles are intended for a single use, but many people use them for several shots. Today’s needles are very fine, and if you were to look at one under a microscope after even a single injection, you’d be surprised by how much damage it sustains. Injecting through clothing (which, contrary to medical myth, is perfectly harmless) dulls needles even more quickly. When you use a dull needle, you raise your risk of small, tearing injuries to the tissue, which in turn raises your risk of scarring. It’s also possible for small needle fragments to break off and be left behind in your skin. And of course, you’ll also be much more likely to experience bruising at the injection site if you’re using a dull needle.

Pen users are more likely to overuse needles, and it’s also common for many users to leave the needle on the pen between shots. This carries two other risks: Some insulins can dry out and crystalize in the needle, fully or partly plugging it, which will affect delivery accuracy. Also, leaving the needle in place can introduce air into the pen reservoir, and an uncapped needle can act like a syphon, causing all the insulin to leak out of the reservoir.

Timing issues

When you take your insulin matters for all insulins, but the most important thing is to be consistent. Long-acting, or basal, insulins should be taken close to the same time every day. While the newer basal insulin analogs are officially “peakless,” if you vary the time of your injections significantly, you can still “stack” your doses, or cause one dose to overlap with the other. The result is too much insulin circulating in your bloodstream, putting you at risk of developing low blood glucose.

Fast-acting insulins work best when injected before a meal, and some studies have shown that taking the insulin 15–30 minutes before starting to eat is highly effective at reducing the high postmeal blood glucose commonly associated with high-carbohydrate meals.

Another important timing issue to consider is the length of time a vial of insulin or pen cartridge is guaranteed to be potent after it is opened or first used (assuming proper storage conditions).

Once you pop the top on a vial or pen, the clock is ticking. Mixed insulins are generally good for only two weeks, and modern insulin analogs last for somewhere around a month. Check the written information that comes with your insulin (or ask your pharmacist) for details on the insulin products you use. If you don’t use up the insulin in a pen or vial by the end of its time frame, you should discard it and start a new one.

Myths and outdated beliefs

Just as insulin formulations and the gear for injecting insulin have changed over the years, so have some of the particulars about using that gear. Here are some updated “truths” about the correct way to inject insulin.

Myth #1

Both the top of the insulin vial or insulin pen (where the needle gets attached) and the skin at the injection site must be cleaned with an alcohol swab to avoid infection.

Fact #1

Most current practice guidelines, including those from the American Diabetes Association and the American Association of Diabetes Educators, no longer advocate for the use of alcohol swabs. There are two reasons alcohol swabs are not recommended: Infection risk from insulin injections is negligible (at least in normal environments – some experts feel hospital environments are riskier), and an alcohol swab is a poor way to sanitize skin in the first place. Soap and hot water are actually more effective.

That said, most Medicare Part D plans and state Medicaid plans continue to cover alcohol pads, and many nursing schools still instruct students to teach diabetes patients to use them. Additionally, syringe and pen needle makers still typically include alcohol swab use in their patient education materials.

For what it’s worth, however, polls of people who have diabetes show that 70 percent of insulin users never use an alcohol swab when injecting.

Myth #2

You must “pinch up” the skin before injecting.

Fact #2

In the old days, it was standard practice to “pinch up” a layer of skin at the injection site to avoid inserting the needle though the subcutaneous fat layer and into the muscle tissue below. Injecting into muscle not only hurts, but it can result in much faster insulin uptake, creating a hypoglycemia risk. But in the old days, the needles were an inch long (or even a little longer)! With most of today’s syringes sporting needle lengths between 6 and 8 millimeters, and with the most popular pen needles a diminutive 4 millimeters in length, pinching has no benefit, and studies show no difference in delivery between pinched and unpinched technique.

Also, if you pinch, you are more likely to bruise. Even very lean people would be highly unlikely to inject into muscle tissue using today’s needles. And most people have plenty of subcutaneous fat to inject into. In fact, many people today now “spread” rather than pinch, stretching the skin out with two fingers to flatten out the target site.

Myth #3

No pain, no gain.

Fact #3

Generally speaking, if it hurts, you aren’t doing it right, or you are using your needles too many times. That said, some areas of the body have more nerve endings than other areas, so if injecting into a certain part of your body hurts, try another location.

Myth #4

Pens are better than syringes because they are more accurate.

Fact #4

While numerous clinical studies indicate that pens reduce dosing errors, increase “compliance,” and raise quality of life scores, there’s more to it than that. On many insurance plans, pens cost more. Also, for people who take large amounts of insulin, a pen may only last for or two three uses. And while pen use in the laboratory may be more accurate, there are many human factors to consider that may introduce error. A syringe used well beats the pants off a pen used poorly.

Myth #5

Insulin needs to be kept in the refrigerator.

Fact #5

Your grandmother’s insulin needed to be kept in the fridge. Maybe. But your opened vial or pen doesn’t. Unopened vials and pens should be kept in the refrigerator to prevent accidental exposure to warm temperatures. But there’s no need to keep an opened vial or pen of insulin in the refrigerator, and you may not want to, because cold insulin stings on injection. All modern insulins can be kept at room temperature.

Check your package insert, because different insulins can tolerate different temperatures. That said, the insulins currently on the market have a top range between 77°–98°F, with most modern insulins having a peak recommended temperature of 86°F. Insulin is most at risk when you travel, so look into getting an insulated bag to protect your insulin when you drive, fly, or travel by other means.

Myth #6

If an injection site bleeds, you have hit a vein and will develop hypoglycemia.

Fact #6

Probably not. Some parts of the body have a lot more capillaries than others. Don’t freak out if you get a little blood. It happens. Some bleeding at the site just means you ruptured a capillary or two, not that you just mainlined your shot.

Myth #7

Insulin will leak out of your body if you use too short a needle.

Fact #7

Insulin that appears on the skin after removing the needle is called backflow. It’s insulin that flows back up the injection pathway after the needle is removed. Mythology holds that it’s more common with shorter needles and fatter people, but neither is true. Backflow happens with both syringes and pens, it happens with all needle gauges (or thicknesses) and lengths, and it happens in both skinny and overweight people.

Backflow is a biological fact. The good news is that studies show it’s a remarkably consistent percentage of the injection. That means the larger the dose, the greater the backflow, but that in turn means that you don’t really need to worry about it — as your dose changes, the loss to backflow is a consistent percentage.

Doing your level best

Sometimes having diabetes feels like juggling feral cats and chainsaws on a tightrope above a tank filled with half-starved piranhas. There are so many things to do, and so many things that can affect your blood glucose level, that it can be overwhelming. But getting the amount of insulin you intend to take into your body is one of the most important things you can do to keep your blood glucose in control.

And that’s what good injection technique is all about. It’s about removing variables that can give you either less or more insulin than you intended to take.

Every once in a while, you should review the basics. Use the information in this article to go over your injection technique, and make sure no “bad” or sloppy habits have crept into your procedures. If you think you’re doing everything “right” but you’re still experiencing blood glucose ups and downs you can’t explain, ask your diabetes educator or physician to watch you use your blood glucose meter and prepare and take an injection. Sometimes personal habits are so ingrained they’re hard to see by yourself, but a second set of eyes can often spot the problem if there is one.

Want to learn more about insulin? Read “Insulin Basics,” “What Does Insulin Do?” and “Insulin: What You Need to Know.”

Originally Published January 21, 2014

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