A CGM for diabetes is a small wearable sensor that measures glucose every few minutes and sends readings to a phone or receiver. Unlike fingersticks, which give one number at a time, continuous glucose monitors show trends, direction arrows, and alerts for highs and lows. Randomized trials have shown CGMs can lower A1C and reduce hypoglycemia for many users.
How a CGM Works
A CGM uses a thin filament placed just under the skin to measure glucose in interstitial fluid — the liquid between your cells. An enzyme on the filament reacts with glucose, and the transmitter converts that signal into a reading. Data is sent wirelessly to your phone or a dedicated receiver.
Because interstitial glucose lags blood glucose by roughly 5-15 minutes, CGM values can differ from a fingerstick during rapid changes. Most sensors now include algorithms that reduce this lag, and manufacturers calibrate devices at the factory so fingerstick calibration is no longer routinely required.
Who Benefits Most From a CGM
The strongest evidence supports CGM use in people taking insulin. Studies published in journals like JAMA and Diabetes Care have shown A1C reductions of 0.3-1.0 percentage points, plus less time in hypoglycemia, when CGMs replace or supplement fingersticks.
Type 1 Diabetes
CGM is now considered standard of care for people with type 1 diabetes by the American Diabetes Association. Alerts for impending lows are especially valuable overnight or during exercise.
Insulin-Using Type 2 Diabetes
Both basal-only and intensive insulin users have shown benefit in trials. Medicare expanded coverage in 2023 to include people with type 2 diabetes on any insulin regimen, reflecting this evidence.
Non-Insulin Type 2 Diabetes
Evidence is growing but mixed. Short-term CGM wear may help with behavior change, and professional CGM (a two-week wear reviewed with a clinician) can guide medication adjustments. Our treatment hub reviews the options in more detail.
Prediabetes and Wellness Use
Over-the-counter CGMs are now available in the U.S. without a prescription. They can show how specific foods affect you, which some people find motivating. However, routine CGM use in people without diabetes has not been proven to improve long-term outcomes, and short-term variability may cause unnecessary anxiety. If you are exploring your metabolic health, our A1C levels resource explains how these numbers fit together.
CGM Systems Available in the United States
Several systems are currently FDA-cleared. Features, accuracy, and cost differ — your clinician can help you choose.
| System | Sensor Life | Calibration | Key Features |
|---|---|---|---|
| Dexcom G7 | 10 days + 12-hour grace | Factory (optional user) | Direct to phone or smartwatch, integrates with multiple insulin pumps |
| FreeStyle Libre 3 | 14 days | Factory | Smallest sensor, 1-minute reads, real-time alerts |
| Eversense E3 | 180 days (implanted) | Twice daily | Long-term implantable sensor, removable transmitter |
| Medtronic Guardian 4 | 7 days | Factory | Pairs with Medtronic insulin pumps for automated delivery |
| Dexcom Stelo (OTC) | 15 days | Factory | Over-the-counter for adults not on insulin, no alerts for lows |
Key Metrics CGM Provides
CGM data is typically summarized in an Ambulatory Glucose Profile (AGP) report. Understanding these metrics helps you have productive conversations with your clinician.
- Time in Range (TIR): percentage of readings between 70-180 mg/dL. ADA targets at least 70% for most adults.
- Time Below Range (TBR): percentage below 70 mg/dL. Target is under 4%, and under 1% below 54 mg/dL.
- Time Above Range (TAR): percentage above 180 mg/dL. Target is under 25%.
- Glucose Management Indicator (GMI): estimated A1C based on CGM data over 14+ days.
- Glycemic Variability (CV): measure of how much your glucose swings; under 36% is the usual goal.
Pros and Cons to Weigh
Potential Benefits
- Fewer fingersticks and more information
- Real-time alerts for highs and lows
- Identification of patterns tied to meals, exercise, sleep, and stress
- Possible reductions in A1C and hypoglycemia with active engagement
- Data sharing with clinicians and family members
Potential Drawbacks
- Cost — even with insurance, copays can add up
- Skin reactions or adhesive issues in some users
- Data overload or anxiety, particularly for new users
- Sensors can occasionally fail or read inaccurately
- Requires a smartphone or compatible receiver, and some dexterity to apply
Getting Started With a CGM
If you think a CGM may help, start by talking with your clinician. They can assess whether you meet coverage criteria, choose a system that fits your needs, and provide training. Most pharmacies and durable medical equipment suppliers now stock the major systems. The NIDDK publishes a patient-friendly overview that is worth reading before your appointment.
Expect a learning curve of two to four weeks. Alerts can initially feel intrusive; most systems let you customize thresholds. Reviewing your AGP report every two weeks with a diabetes educator or clinician can turn raw data into meaningful adjustments.
The Bottom Line
A CGM for diabetes transforms a single snapshot into a continuous movie of your glucose, offering insights that fingersticks cannot provide. For many people on insulin, the evidence for improved control and safety is strong. For others, the decision is more individual and should be made with your clinician, weighing cost, lifestyle, and how you will use the information.
