What a continuous glucose monitor does
A continuous glucose monitor (CGM) is a small sensor worn on your skin that measures your blood sugar every few minutes throughout the day and night. Instead of pricking your finger for a single reading, a CGM gives you a stream of data—usually checking glucose levels every 5 to 15 minutes depending on the brand. The sensor sits just under the skin, usually on your abdomen or arm, and sends readings wirelessly to a receiver or smartphone app.
The sensor itself is about the size of a postage stamp and stays on for 7 to 14 days before you replace it. You insert it yourself using a small applicator that looks like a pen—it takes a few seconds and most people report minimal discomfort. The real value is not in the insertion but in what happens after: you see patterns in how your blood sugar responds to food, exercise, stress, and sleep without having to test constantly.
Key Takeaways
- CGMs measure glucose every 5 to 15 minutes and show trends on your phone or receiver, letting you see how your body responds to meals and activity in real time.
- The main brands—Dexcom, FreeStyle Libre, and Medtronic—differ in sensor wear time (7 to 14 days), whether they need fingerstick calibration, and what they cost out of pocket.
- Insurance often covers CGMs for people with type 1 diabetes or insulin-using type 2 diabetes, but coverage for type 2 diabetes without insulin varies widely by plan and state.
- Costs range from roughly $35 to $100 per sensor if you pay out of pocket, plus receiver or app fees, and sensors must be replaced regularly rather than used indefinitely.
- CGMs work best when you actively check the data and act on patterns; simply wearing one without looking at readings provides little benefit.
How the sensor and readings actually work
The sensor contains a tiny electrode that measures glucose in the fluid just under your skin (called interstitial fluid). This fluid lags slightly behind your blood glucose—usually by 5 to 15 minutes—so CGM readings are not instantaneous snapshots but a delayed reflection of what is happening in your bloodstream. That lag matters: if your blood sugar is dropping fast, the CGM may show a normal reading while your actual blood glucose is already lower.
The sensor sends data wirelessly to either a dedicated receiver device (which looks like a small pager) or directly to your smartphone via Bluetooth. Most newer systems use your phone as the receiver, which means you carry one device instead of two. The app displays your current glucose, a trend arrow showing whether levels are rising or falling, and a graph of your readings over the past few hours. Some systems also send alerts to your phone or smartwatch when glucose goes too high or too low.
Accuracy is generally good but not perfect. CGMs are typically within 10 to 15 percent of a lab blood test, which is close enough for most decisions but not precise enough to replace fingerstick testing in all situations. Some systems require occasional fingerstick calibration (usually once or twice a day) to stay accurate; others do not. The newer Dexcom G7 and FreeStyle Libre 3, for example, do not require fingerstick calibration, while older models do.
The three major CGM brands and how they differ
Dexcom makes the G6 and G7 systems. The G7 sensor lasts 10.5 days, requires no fingerstick calibration, and sends readings to your phone every 5 minutes. Dexcom also allows you to share readings with up to five people (useful if a parent or partner wants to monitor your glucose), and the system integrates with some insulin pumps so the pump can adjust insulin automatically based on CGM readings. The receiver is your smartphone; there is no separate device to carry.
FreeStyle Libre (made by Abbott) offers the Libre 2 and Libre 3. The Libre 2 lasts 14 days and requires no calibration; the Libre 3 also lasts 14 days and is smaller. Both use your phone as the receiver. The Libre systems are generally less expensive out of pocket than Dexcom, and the longer wear time means fewer sensor changes per year. The trade-off is that Libre does not integrate with as many insulin pumps, and some users report slightly less frequent data sharing (the Libre 2 scans every 8 hours unless you use a reader device).
Medtronic makes the Guardian system, which is designed to work with Medtronic insulin pumps. The sensor lasts 7 days and requires fingerstick calibration twice a day. Medtronic systems are often covered by insurance if you are already using a Medtronic pump, but they are less commonly used as standalone systems compared to Dexcom or Libre. The Guardian can automatically suspend insulin delivery if your glucose drops too low, a feature called predictive low glucose suspend.
What insurance covers and what you pay out of pocket
Insurance coverage depends heavily on your diagnosis and your specific plan. Most insurance plans cover CGMs for people with type 1 diabetes, especially if you use insulin. Coverage for type 2 diabetes is more variable: some plans cover it only if you use insulin, others cover it for anyone on certain medications, and some do not cover it at all. A few states have passed laws requiring coverage for type 2 diabetes, but this is not universal.
If your plan does cover a CGM, your out-of-pocket cost is usually a copay per sensor—typically $0 to $50 depending on your plan—plus any deductible you have not met. If your plan does not cover it, you pay the full retail price. A single sensor costs roughly $35 to $100 depending on the brand and where you buy it; since you replace sensors every 7 to 14 days, annual sensor costs range from roughly $1,800 to $5,000 before any other fees.
Some manufacturers offer patient assistance programs that reduce the cost if you do not have insurance or your insurance does not cover CGMs. Dexcom, FreeStyle, and Medtronic all have these programs, though may be able to access and the discount amount vary. It is worth calling the manufacturer directly to ask what programs exist for your situation. Some endocrinologists' offices also have samples or can help you navigate coverage questions.
Real advantages of wearing a CGM
The biggest advantage is visibility into patterns you cannot see with fingerstick testing. A single blood sugar reading tells you what your glucose is right now; a CGM shows you what happens after you eat a specific food, how exercise affects your levels hours later, or how stress and sleep influence your glucose overnight. Many people discover that foods they thought were safe actually spike their blood sugar, or that a particular type of exercise drops their glucose more than they expected.
For people using insulin, a CGM makes it easier to adjust doses based on real data rather than guessing. If you can see that your blood sugar is trending up before a meal, you can take insulin earlier or take a slightly larger dose. If you see that a particular snack does not spike your glucose much, you can eat it without worry. This feedback loop helps you make better decisions over time.
CGMs also reduce the burden of testing. Instead of pricking your finger 4 to 10 times a day, you wear a sensor and check your phone. For people with needle anxiety or those who find frequent testing exhausting, this is a meaningful quality-of-life improvement. The continuous data also means you are less likely to miss dangerous lows at night, since the CGM can alert you if your glucose drops below a threshold while you sleep.
Real limitations and drawbacks
CGMs measure glucose in interstitial fluid, not blood, so readings lag behind your actual blood glucose by 5 to 15 minutes. This matters most when your glucose is changing rapidly—during or right after exercise, or if you are treating a low. In those moments, your CGM reading may not reflect what is actually happening in your bloodstream, and you may need to fingerstick to confirm.
The sensors are not waterproof on all models, though most can handle brief water exposure like showering. Some people find the adhesive irritates their skin, especially if they wear the sensor in the same spot repeatedly. Sensor failures happen occasionally—a sensor may stop reading accurately or stop transmitting data before the wear period ends, and you have to replace it early. Some insurance plans do not cover replacement sensors if the original one failed, so you may pay out of pocket.
A CGM only works if you actually look at the data and act on it. Simply wearing a sensor without checking your readings provides almost no benefit. Many people find the constant stream of glucose information stressful at first, especially if they see numbers they did not expect. Some develop anxiety about eating anything that might spike their glucose. The device requires discipline to use well, and it is not a substitute for understanding how your body responds to food and activity.
Costs and what affects the total price
The total cost of using a CGM includes the sensor itself, any receiver device, and sometimes a subscription or app fee. Here is what varies:
| Cost Component | Typical Range | What Affects It |
|---|---|---|
| Single sensor (retail) | $35–$100 | Brand, where you buy it, whether you have insurance |
| Annual sensor cost (out of pocket) | $1,800–$5,000 | Sensor wear time (7 vs. 14 days) and brand |
| Receiver device | $0–$400 | Most systems use your phone; separate receivers cost more |
| Insurance copay per sensor | $0–$50 | Your specific plan and deductible |
If you have insurance that covers CGMs, your actual out-of-pocket cost is usually much lower—often just a copay per sensor. If you do not have insurance, the full retail cost adds up quickly. Some people buy sensors in bulk or use discount programs like GoodRx to reduce the per-sensor price, though this is less common now that more insurance plans cover CGMs.
Frequently Asked Questions
Do I need a prescription to buy a CGM?
Yes. All CGM systems require a prescription from a doctor. Your doctor will need to confirm your diagnosis (type 1 or type 2 diabetes, or prediabetes in some cases) and write a prescription for the specific brand. You then order through the manufacturer or a pharmacy, and insurance processes the claim if you have coverage.
Can I use a CGM if I have type 2 diabetes and do not take insulin?
It depends on your insurance plan and your doctor. Some plans cover CGMs only for insulin users; others cover them for anyone with type 2 diabetes. A few states require coverage regardless of insulin use. Your doctor can check your insurance coverage before writing a prescription, or you can call your insurance company directly and ask what their policy is for type 2 diabetes without insulin.
How often do I have to change the sensor?
Every 7 to 14 days depending on the brand. Dexcom G7 lasts 10.5 days, FreeStyle Libre lasts 14 days, and Medtronic Guardian lasts 7 days. You remove the old sensor, throw it away, and insert a new one. The process takes a few minutes and you do it yourself at home.
What happens if the sensor falls off or stops working?
If a sensor falls off before the wear period ends, you can reapply it if it is still sticky, or you insert a new one. If a sensor stops reading accurately or stops transmitting, you replace it. Some insurance plans cover replacement sensors; others charge you a copay or make you pay out of pocket. Ask your insurance or the manufacturer about their replacement policy before you start.
Can a CGM replace fingerstick testing completely?
Not entirely. CGMs are accurate enough for most daily decisions, but they lag behind your blood glucose and are not as precise as a fingerstick test. Most doctors recommend keeping a fingerstick meter on hand for situations where you need an immediate, accurate reading—like when your CGM shows a low but you do not feel low, or when you are troubleshooting an unexpected high.