A plain-English guide to continuous glucose monitors — how they work, what they tell you that a standard blood test can't, and whether you might benefit from one.
You may have noticed a small oval patch on someone's upper arm at the gym or at the beach and wondered what it was. Or your doctor mentioned a CGM and you nodded along without quite knowing what they meant. Or you've been reading about Ozempic and Wegovy and kept seeing references to "time in range" and "glucose spikes."
A CGM — continuous glucose monitor — is the device behind all of those things. And it represents one of the most meaningful shifts in how people understand and manage blood sugar in the past twenty years.
A CGM is a small wearable sensor that measures your blood sugar automatically, every few minutes, around the clock — sending the data to your phone so you can see what's happening in real time rather than waiting for a lab result.
Traditional blood sugar testing requires a fingerstick — you prick your finger, place a drop of blood on a test strip, and get a single glucose reading at that one moment in time. It works, but it's a snapshot taken once or twice a day, and it misses everything in between.
A CGM is different in two fundamental ways: it measures continuously, and it doesn't require blood.
Here's what happens when you apply one:
The sensor is waterproof, worn continuously, and typically lasts 10 to 15 days before you swap it out. No fingersticks required during normal use.
A standard blood glucose test tells you your level at a single moment. Your A1C tells you your average over three months. A CGM tells you everything in between — the shape of the curve, not just two points on it.
This reveals things that are invisible to conventional testing:
The most important concept CGMs introduced is time in range (TIR) — the percentage of the day your glucose spends within a target zone, typically 70 to 180 mg/dL for most adults with diabetes.
The American Diabetes Association recommends that most adults with diabetes aim for more than 70% time in range — meaning their glucose is within target for at least 16–17 hours of every day.
For healthy adults without diabetes, research from the Framingham Heart Study found that people without diabetes spend about 87% of their time in the tighter range of 70 to 140 mg/dL, with less than 15 minutes per day above 180 mg/dL. That's a useful reference point for anyone wearing an OTC CGM for wellness purposes.
Time in range is more informative than A1C alone because two people can have the same A1C but very different glucose patterns — one spending most of the day near-normal with occasional highs, the other spending the day swinging between lows and highs that average out to the same number. The CGM sees the difference. The A1C doesn't.
More than 16–17 hours per day in the 70–180 mg/dL range.
In 70–140 mg/dL range per Framingham study data.
Less than 1 hour per day below 70 mg/dL is the ADA target.
| Test | What it measures | Timeframe | Best for |
|---|---|---|---|
| Fingerstick glucose meter | Blood glucose right now | A single moment | Checking before/after meals, confirming a CGM reading, insulin dosing |
| A1C blood test | Average glucose over ~90 days | Past 2–3 months | Diagnosing diabetes, monitoring long-term control, checking treatment effectiveness |
| CGM | Glucose continuously, day and night | Every 1–5 minutes, 10–15 days | Understanding patterns, identifying spikes, overnight monitoring, real-time feedback |
These three tests complement rather than replace each other. A CGM doesn't make your A1C irrelevant; a high A1C is still the standard for diagnosing and monitoring diabetes. But the CGM explains why the A1C is what it is, and shows you what to change. Use our A1C to eAG calculator to see how your A1C maps to an average daily glucose number.
CGMs were originally developed for type 1 diabetes, where blood sugar can change rapidly and unpredictably. For people using insulin, especially those with automated insulin delivery ("closed-loop") systems, a CGM isn't a convenience — it's essential. Real-time glucose data allows the insulin pump to automatically adjust delivery. The ADA now considers CGM the standard of care for everyone on insulin therapy.
This is the largest and fastest-growing group of CGM users. Most people with type 2 diabetes managed by diet, exercise, or oral medications don't have the severe lows that make CGM medically urgent — but the visibility into post-meal spikes, time in range, and the effect of lifestyle choices is enormously valuable. The arrival of over-the-counter CGMs in 2024 (Stelo and Lingo) dramatically reduced the cost and access barriers for this group. See our Stelo vs. Lingo comparison for the options available without a prescription.
A CGM worn for two to four weeks can be a powerful motivational tool — showing exactly which foods spike your blood sugar, confirming that a 20-minute walk after dinner meaningfully reduces the post-meal peak, and providing concrete data to discuss at your next doctor's appointment.
A growing category. Lingo, Abbott's OTC CGM, is explicitly designed for people without diabetes who want to understand their metabolic health. Researchers, athletes, biohackers, and people with family history of diabetes are among the early adopters. The evidence that short-term CGM use durably changes dietary behavior in healthy people is still emerging — but for many, the data is revelatory.
A CGM is a natural companion to Ozempic, Wegovy, Mounjaro, or Zepbound. It shows you the effect the medication is having on your post-meal glucose in real time — the blunted spikes, the lower peaks, the extended time in range. For people who are motivated by data, seeing the medication work can improve adherence. See our GLP-1 comparison guide.
Until 2024, all CGMs in the US required a prescription. That changed when Dexcom and Abbott received FDA clearance for over-the-counter devices.
| Prescription CGMs | OTC CGMs (Stelo, Lingo) | |
|---|---|---|
| Who they're for | Anyone with diabetes, including insulin users | Adults not on insulin |
| Prescription needed | Yes | No |
| Insurance coverage | Often covered, especially for T1D | Generally not covered; HSA/FSA eligible |
| Low glucose alarms | Yes — essential for insulin users | No — not designed for hypoglycemia detection |
| Cost | Varies; $0–$200/month with insurance | ~$84–$89/month |
| Examples | Dexcom G7, Abbott Libre 3 Plus | Dexcom Stelo, Abbott Lingo |
If you use insulin, an OTC CGM is not appropriate — it lacks the alarms and clinical accuracy features you need. Talk to your doctor about a prescription device. If you have type 2 diabetes managed without insulin, or no diabetes at all, an OTC device is a legitimate and accessible option.
Most people are surprised by how unobtrusive a CGM is. The insertion pinches briefly, then nothing — most wearers forget it's there within a few hours. The sensor is waterproof for showers and swimming (within the limits specified by each device). The adhesive holds well for most people, though active users sometimes use an extra adhesive patch over the top.
The first 30–60 minutes after applying a new sensor are the warm-up period, during which readings are less accurate. After that, the data is reliable for the wear duration of the sensor.
One thing to know: CGM readings can lag behind blood glucose by about 5–10 minutes because interstitial fluid responds slightly slower than blood. This is usually irrelevant for lifestyle monitoring but matters if you're making insulin dosing decisions — always confirm with a fingerstick if your CGM and symptoms don't agree.
CGM accuracy is measured by Mean Absolute Relative Difference (MARD) — the average percentage gap between a CGM reading and a simultaneous reference blood glucose measurement. Lower is more accurate.
Both OTC devices are well within clinically acceptable accuracy for wellness monitoring. For insulin dosing or clinical decision-making, a prescription device with proven accuracy in rapid-change situations is preferable.
A CGM is not a replacement for your A1C test or your doctor. It's a window — the first tool that shows blood sugar as a continuous story rather than an occasional data point. For people managing diabetes or prediabetes, that story is full of patterns that motivate real change: the meal that quietly spikes your glucose every Tuesday, the walk that brings it back down, the stressful Monday that pushes it up before you've eaten anything.
For most of the past decade, that window was available only to people with prescriptions and insurance coverage. The OTC devices that launched in 2024 opened it to anyone willing to pay ~$89 a month — which is a genuine shift in what's possible for self-directed health management.