A CGM (continuous glucose monitor) is a small wearable device that tracks your blood sugar levels around the clock, giving you a reading every few minutes without finger pricks. It works by measuring glucose in the fluid just beneath your skin, then wirelessly sending that data to your phone or a small receiver so you can see your levels in real time and spot trends throughout the day.
How a CGM Works
Every CGM system has three basic parts. The first is a tiny sensor, about the width of a few human hairs, that you insert just under your skin with a spring-loaded applicator. This sensor sits in the interstitial fluid (the liquid surrounding your cells) and continuously measures glucose concentration. The second part is a transmitter, either built into the sensor or snapped on top of it, that wirelessly sends glucose data to the third component: a smartphone app, a dedicated receiver, or an insulin pump display.
Because the sensor reads glucose from interstitial fluid rather than directly from your blood, there’s a slight delay between what your blood sugar is actually doing and what the CGM reports. This lag can be up to 15 minutes but is typically less. The gap is most noticeable when your glucose is changing rapidly, like right after a meal or during intense exercise. For most of the day, the readings closely match what a finger-prick meter would show.
Who Benefits Most From a CGM
CGMs were originally designed for people with type 1 diabetes, and that’s still where they deliver the most value. Because type 1 involves constant insulin dosing decisions, having a continuous stream of glucose data helps you (or an insulin pump) choose the right dose at the right time. Real-time alerts can warn you before your blood sugar drops dangerously low or climbs too high, which is especially important overnight when you can’t check manually.
For people with type 2 diabetes who take insulin, a CGM serves as an early warning system for low blood sugar episodes. The benefit becomes less clear for people with type 2 who aren’t on insulin or other medications that can cause lows. As Harvard endocrinologist David Nathan has noted, the vast majority of people with type 2 diabetes don’t need one on a regular basis because their medications don’t typically cause dangerous glucose drops. That said, some people with type 2 use a CGM for short periods to learn how specific foods and habits affect their blood sugar, even if they don’t need one long-term.
Popular Devices Compared
The two most widely used CGMs right now are the Dexcom G7 and the FreeStyle Libre 3. Both are about the size of a coin, attach to the back of your upper arm, and send readings to your phone. The practical differences come down to a few specs:
- Sensor wear time: The Dexcom G7 lasts 10 days before you replace it. The FreeStyle Libre 3 lasts 14 days.
- Warm-up time: After applying a new sensor, the Dexcom G7 needs about 30 minutes before it starts giving readings. The Libre 3 takes about 60 minutes.
- Accuracy: Both devices are highly accurate. The Dexcom G7 has a mean error rate of about 8.2% in adults, while the Libre 3 comes in around 7.9% to 8.9% depending on the study conditions. In practical terms, both are accurate enough to make insulin dosing decisions without confirming with a finger prick.
There’s also a longer-term option called the Eversense E3, which uses a sensor implanted under the skin by a doctor. It lasts up to 180 days (six months) before needing replacement, which means far fewer sensor changes per year. The trade-off is that both insertion and removal require an in-office procedure.
What the Data Actually Tells You
A CGM gives you more than just a current number. It shows a continuous graph of where your glucose has been, where it is now, and which direction it’s heading. An upward arrow means your blood sugar is rising; a downward arrow means it’s falling. This trend information is often more useful than the number itself because it lets you act before a high or low happens rather than reacting after the fact.
The key metric most providers focus on is called Time in Range, or TIR. This is the percentage of the day your glucose stays between 70 and 180 mg/dL. The general target for most adults with diabetes is at least 70% of the day in range, which works out to roughly 17 hours out of 24. Equally important is minimizing time below range: less than 4% of the day between 54 and 69 mg/dL, and less than 1% below 54 mg/dL. These thresholds give you a much fuller picture of glucose control than a single fasting number or even a lab test like A1C, because they capture what’s happening after meals, during sleep, and during activity.
Where to Place the Sensor
The back of the upper arm is the most common placement site and the one used in accuracy studies for both major devices. Other options include the abdomen (at least a few inches from the belly button), outer thighs, upper buttocks, hips, and lower back, though you should check your specific device’s instructions since not every brand is approved for every location.
Rotating your sensor site each time you apply a new one is important. Use a simple box pattern on one area, moving a few inches with each new sensor, then switch to the other side of your body. Avoid spots where the sensor could be compressed for long periods, like areas you sleep on, sit on, or cover with a tight waistband. If you also use an insulin pump, keep the CGM sensor at least three inches away from the pump’s infusion site.
Dealing With Skin Irritation
The most common side effect of wearing a CGM isn’t related to the sensor itself but to the adhesive that holds it in place. Redness, itching, and rashes at the application site affect a significant number of users, particularly those who have eczema, asthma, or general allergy sensitivity.
A barrier product applied before the sensor can make a big difference. Liquid barriers like IV Prep, Cavilon No Sting Barrier, or SurePrep create a thin protective film between your skin and the adhesive. You apply them to the area where the patch will sit, leave a small gap where the sensor needle will actually enter the skin, and let the product dry completely before placing the sensor. For people who need extra hold in addition to skin protection, products like Skin Tac provide stronger adhesion. Mastisol is the strongest option but is more likely to cause irritation on its own, so it’s usually a last resort.
Cost and Insurance Coverage
CGM coverage varies widely depending on your insurance. Medicare covers CGMs as durable medical equipment, but with specific requirements: you need a diabetes diagnosis, a prescription from your provider, training on how to use the device, and documentation that the CGM is being prescribed to improve glucose control for someone who is on insulin therapy or has a history of problematic low blood sugar episodes. People with type 2 diabetes who don’t use insulin can still qualify under Medicare if they have documented recurring episodes of significant hypoglycemia.
Private insurers generally follow similar logic, covering CGMs most readily for people on intensive insulin regimens. Without insurance, the out-of-pocket cost for sensors alone typically runs several hundred dollars per month, though over-the-counter options like the FreeStyle Libre 2 have brought prices down for people willing to pay cash. If you’re considering a CGM, your provider’s office can usually check your specific coverage before writing the prescription.