What is a CGM? A plain English guide.
A continuous glucose monitor is a small sensor you wear on your arm or abdomen that reads your blood sugar around the clock and sends it to a phone app. It replaces the need to prick your finger every time you want a reading, and it shows you what your numbers actually do across a day, not just a single moment.
What a CGM actually is
"CGM" stands for continuous glucose monitor. The name does most of the work: it is a device that monitors your glucose continuously, instead of measuring it at one moment in time the way a fingerstick does.
Physically, a CGM has three small parts:
- A sensor about the size of a coin, with a hair-thin filament that sits just under your skin.
- A transmitter built into the sensor housing that sends readings to your phone over Bluetooth.
- An app on your phone that displays your current number, your trend over time, and gentle insights.
You apply the sensor yourself with a single-use applicator. It stays on for the full wear cycle (typically 10 to 15 days depending on the model), and during that time it quietly reports a new glucose value every few minutes.
How the sensor works
The filament under your skin sits in the interstitial fluid, which is the fluid between your cells, not in your blood. The sensor uses a tiny electrochemical reaction to measure how much glucose is in that fluid, and that value is closely related to (but not identical to) the glucose in your blood. For a clinical overview of how the technology works, see NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases) on continuous glucose monitoring. Cleveland Clinic also publishes a useful patient-focused explainer.
Because interstitial glucose lags blood glucose by roughly 5 to 15 minutes, your CGM reading after a fast meal or quick exercise will trail what a fingerstick would show in the same moment. Once your glucose settles, the two values track closely. Cleveland Clinic on continuous glucose monitoring covers the interstitial-fluid mechanism in more detail.
Readings transmit to your phone via Bluetooth. There is no needle in the traditional sense: the filament is flexible, very thin, and most users report no sensation during normal wear.
Who it is for (and who it is not)
In the United States today, two distinct categories of CGM exist, with different intended audiences. Understanding which category fits you is the most important question to answer before going further.
Over-the-counter (OTC) CGMs
OTC CGMs are designed for adults 18 years of age or older who are not on insulin therapy. That includes a lot of people: adults with prediabetes, adults with type 2 diabetes who manage with metformin or GLP-1 medications, and adults curious about metabolic health who do not have a diagnosis. These devices are cleared by the FDA for purchase without a prescription. See the FDA clearance announcement for the official notice.
Prescription CGMs
Prescription CGMs are designed for people who need clinical-grade monitoring, especially anyone on insulin therapy. These devices can be used as part of insulin dosing decisions and integrate with insulin pumps. They require a prescription and typically a qualifying diagnosis for insurance coverage. The American Diabetes Association maintains the canonical patient-facing summary of how CGMs are used in care.
If you use insulin, including basal-only insulin, please do not rely on an OTC CGM. You need a clinical-grade device managed by your healthcare provider, not a consumer one. None of this is medical advice; loop in your doctor before making changes.
OTC vs prescription in 2026
For most of the history of continuous glucose monitoring, you needed a prescription. That changed when the FDA cleared the first integrated CGMs specifically as over-the-counter devices for non-insulin users.
The practical upshot for our reader: if you are an adult, not on insulin, and want to understand your own glucose patterns, you no longer need a doctor's note. You can buy a sensor online directly. We will get into the specific products and how to choose between them in separate articles.
What you'll see on day one
Your CGM app, regardless of brand, will show roughly the same things:
- A big current number in milligrams per deciliter (mg/dL in the US) or millimoles per liter (mmol/L outside the US).
- A trend arrow showing whether your glucose is steady, climbing, or falling, and how fast.
- A line graph of your glucose over the last few hours.
- A time in range percentage, which is the share of the day your glucose was inside a target window.
- An average for the day, the week, or longer.
For most non-diabetic adults, glucose sits around 70 to 99 mg/dL when fasting and stays under about 140 mg/dL one to two hours after a normal meal, per the NIDDK diabetes overview. Your own baseline is more useful than a textbook number, so spend a day or two just observing before you try to change anything.
What a CGM does not do
This is the section that should temper expectations. CGMs are remarkable tools, but they are not magic.
- A CGM does not diagnose diabetes. Diagnosis is made by a clinical lab test (A1C, fasting plasma glucose, or oral glucose tolerance test) ordered and interpreted by a healthcare provider.
- It is not for insulin dosing. OTC CGMs are explicitly not intended for insulin therapy decisions. People who need that capability use prescription CGMs under medical supervision.
- It does not see in real time. Sensor readings lag blood readings by a few minutes because they measure interstitial fluid, not blood. This is rarely a problem for trend monitoring; it would be a problem for medication dosing.
- It does not replace your doctor. Patterns the CGM reveals are conversation starters with your provider, not substitutes for that conversation.
Thinking about trying one
We will not push you to buy anything. The honest framing is this: if you are curious about how your own body responds to your specific meals, your specific sleep, your specific stress, a CGM is the most personal information tool that exists for that question. Two weeks of data will teach you more about your metabolism than a year of fasting blood tests.
If you do not want a sensor on your arm, that is a completely valid choice. An A1C from your annual physical, paired with attention to how you feel after meals, will get you most of the way to better metabolic awareness. If you want to see what an A1C number translates to in the everyday glucose units you would see on a sensor, our A1C to average glucose converter does it instantly.
We are intentionally not recommending a specific product on this page. When we eventually publish product comparisons, you will see them in a separate section of the site, with a clear disclosure if any of the links earn us a commission. For now, this site has no affiliate links, no ads, and no commercial relationships of any kind.
For deeper reading on continuous glucose monitoring, four authoritative starting points: NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases), Cleveland Clinic, American Diabetes Association, and CDC (Centers for Disease Control and Prevention) diabetes. All four are non-commercial sources widely used by clinicians.
Common questions
Do I need a prescription for a CGM?
Not always. In the United States, two over-the-counter continuous glucose monitors are now available for adults 18 and older who are not on insulin. Prescription CGMs still exist for people who need clinical-grade monitoring, especially those on insulin therapy.
Does a CGM hurt to apply?
Most people report a brief pinch from the applicator and then no sensation. The filament that sits under your skin is hair-thin and is designed to be barely noticeable during normal wear.
How accurate is a CGM compared to a fingerstick?
Modern CGMs are well-validated for tracking trends and patterns, with sensor readings typically lagging blood readings by roughly 5 to 15 minutes. They are not intended for insulin dosing decisions. [VERIFY: cite MARD values from a clinical accuracy study]
How long does one sensor last?
Most CGMs on the market today last between 10 and 15 days per sensor. After that the sensor stops transmitting and you apply a new one.
Can I wear a CGM during exercise, showering, or sleeping?
Yes. CGMs are designed to be worn continuously. They are water-resistant to the manufacturer's specification and stay attached during normal daily activities including exercise and sleep.
This article is educational and not medical advice. If you have questions about whether a CGM is right for you, or about what your numbers mean, please bring them to your healthcare provider, who can see the full picture.
References
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Continuous glucose monitoring. niddk.nih.gov/health-information/diabetes/overview/managing-diabetes/continuous-glucose-monitoring.
- Cleveland Clinic. Continuous glucose monitoring (CGM). my.clevelandclinic.org/health/articles/continuous-glucose-monitoring-cgm.
- American Diabetes Association. Continuous glucose monitors. diabetes.org/advocacy/cgm-continuous-glucose-monitors.
- Centers for Disease Control and Prevention. Diabetes. cdc.gov/diabetes.
- U.S. Food and Drug Administration. "FDA Clears First Over-the-Counter Continuous Glucose Monitor." March 5, 2024. fda.gov.
- [VERIFY: add a peer-reviewed citation on CGM accuracy / MARD, e.g. a recent meta-analysis from Diabetes Technology & Therapeutics or Journal of Diabetes Science and Technology]
- [VERIFY: add the manufacturer specification sheet you reference for OTC sensor wear time and warm-up periods]
Last reviewed: . We update this article when guidance, devices, or regulatory status meaningfully change.
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