Imagine knowing your blood sugar is dropping while you’re asleep-before you even feel dizzy. No finger pricks. No guessing. Just an alert on your phone telling you to grab a snack. That’s what a continuous glucose monitor does for millions of people with diabetes today.
How CGMs Actually Work
A continuous glucose monitor doesn’t measure blood sugar directly. Instead, it reads glucose in the fluid between your cells-called interstitial fluid. A tiny sensor, about the size of a coin, is inserted just under your skin, usually on your belly or arm. Inside that sensor is a thin filament coated with glucose oxidase enzyme. When glucose in the fluid touches it, a chemical reaction creates a tiny electric current. The strength of that current tells the device how much glucose is around. That signal gets sent wirelessly to a receiver or your smartphone every 1 to 5 minutes. Modern devices like the Dexcom G7 and Abbott FreeStyle Libre 3 don’t need you to calibrate them with fingersticks anymore. They’re factory-calibrated and ready to go right out of the box. The FreeStyle Libre 3 even measures glucose every single minute, giving you a real-time stream of data. But here’s the catch: there’s a delay. Because the sensor is reading interstitial fluid, not blood, it lags behind what’s happening in your bloodstream by 5 to 20 minutes. That means if your blood sugar plummets during a sprint or after eating, the CGM might not catch it instantly. That’s why experts say the trend arrow-the little up, down, or flat line next to your number-is often more useful than the number itself. It tells you if your glucose is rising fast, falling fast, or staying steady.Who Uses CGMs-and Why
The biggest group using CGMs are people with type 1 diabetes. For them, it’s not a luxury-it’s a lifeline. A 2022 analysis of 32 clinical trials showed that CGM users had 31% fewer severe low blood sugar episodes. That’s huge. Severe hypoglycemia can lead to seizures, unconsciousness, or even death. With a CGM, alarms wake you up before your blood sugar crashes too low. People with type 2 diabetes who take insulin multiple times a day also benefit. Many of them don’t realize how much their sugar swings after meals or during stress. A CGM shows those patterns clearly. One user on Reddit said, “I thought I was eating healthy-until my Libre showed my sugar spiked to 240 after a salad with nuts and olive oil.” That kind of insight helps people adjust food choices, timing, and medication. Even non-diabetics are starting to use CGMs. Athletes track how carbs affect their energy. People with prediabetes use them to test how lifestyle changes impact their numbers. But insurance rarely covers CGMs for non-diabetics, and the cost-up to $400 a month without coverage-makes it a tough sell for most.CGM Brands Compared
There are three main types of CGMs on the market, each with different strengths.- Dexcom G7: Measures every 5 minutes. Needs no fingersticks for calibration. Comes with predictive alerts that warn you 20 minutes before a low. Costs about $399/month without insurance. Accuracy is strong, with a MARD (mean absolute relative difference) of 9.1% compared to blood tests.
- Abbott FreeStyle Libre 3: Smallest and thinnest sensor on the market. Measures every minute. No calibration needed. Costs $110/month for 3 sensors. Doesn’t have automatic insulin pump integration or low-suspend features. But it’s easy to wear, and the app is simple. MARD is just 7.9%-the most accurate of the major CGMs.
- Eversense E3: Implanted under the skin. Lasts up to 6 months. Requires a minor procedure to insert and remove. Vibrates on your skin when your glucose is too high or low. No transmitter to tape on. Costs more upfront but saves money over time. Only available in the U.S. and a few other countries.
Most users pick based on cost, convenience, and features. If you want alarms while you sleep, Dexcom is the go-to. If you want the cheapest, easiest option with no daily hassles, Libre 3 wins. If you hate wearing devices on your skin, Eversense is the answer.
The Real Benefits Beyond Numbers
The biggest win isn’t just the data-it’s the peace of mind. People who use CGMs report sleeping better. They feel less anxious about going out to dinner or exercising. Parents of kids with type 1 diabetes say they can finally leave the house without a panic plan. One study found that CGM users had 24% fewer hospital visits related to diabetes. That’s not just about avoiding lows. It’s about catching highs before they turn into ketoacidosis. It’s about seeing how stress, sleep, or a bad night affects your numbers over days-not just one snapshot at your doctor’s office. The trend arrows make all the difference. Seeing your glucose falling at 2% per minute means you can eat 15 grams of carbs now. Waiting until your number hits 65 means you’re already shaky, sweaty, and confused. That’s the real power of continuous data.Problems and Pitfalls
CGMs aren’t perfect. Skin irritation is the #1 complaint. The adhesive on Dexcom sensors can cause redness, itching, or rashes, especially if you sweat a lot or have sensitive skin. Many users swear by Skin Tac wipes or Opsite Flexifix tape to make the sensor stick better. False alarms are another issue. Some users report their CGM showing a low during exercise-even when their fingerstick says 100. That’s because of the lag. Your blood sugar drops fast during a workout, but the interstitial fluid hasn’t caught up yet. The CGM reads it as low, but your body isn’t there yet. That’s why experts say: if you feel symptoms, always check with a fingerstick. Some medications interfere. Acetaminophen (Tylenol) can cause falsely high readings on most CGMs. If you take it for a headache, your glucose might look like it’s spiking-when it’s not. Always check the manufacturer’s safety notice for drug interactions. And then there’s information overload. Some older adults or people with anxiety find the constant stream of numbers overwhelming. One doctor put it bluntly: “If you’re already stressed about your health, a CGM might make it worse.”Who Shouldn’t Use a CGM?
Not everyone needs one. If you manage diabetes with diet and oral meds only, and you rarely have lows, a CGM might be overkill. If you’re uncomfortable with technology or don’t want to check your phone every hour, it might not fit your life. Also, if you can’t afford it and don’t qualify for insurance coverage, it’s a heavy financial burden. In the U.S., Medicare now covers CGMs for insulin users-but not flash monitors like Libre unless you’re on insulin. In the UK, the NHS gives FreeStyle Libre 2 to over half a million people for free. In other countries, access varies wildly.
What’s Next for CGMs?
The next big leap? Non-invasive sensors. Companies like Glucowise are testing optical devices that read glucose through the skin without a needle. Apple is rumored to be working on a glucose-sensing smartwatch. If that happens, CGMs could become as common as fitness trackers. Hybrid closed-loop systems-also called artificial pancreases-are already here. Devices like Tandem’s Control-IQ use CGM data to automatically adjust insulin delivery. They predict lows and suspend insulin before they happen. Early users report spending 70% more time in the target glucose range. Soon, CGMs might track more than glucose-ketones, lactate, even stress hormones. That could help people with type 1 diabetes avoid diabetic ketoacidosis before it starts.Getting Started
If you’re considering a CGM, talk to your doctor first. Most require a prescription. Ask about insurance coverage. In the U.S., check if your plan covers Dexcom, Abbott, or Medtronic. In the UK, ask if you qualify for NHS funding. Most systems come with apps that walk you through setup. It takes about 15 minutes to insert the sensor and pair it with your phone. Most people get comfortable with trend arrows and alerts within a few days. Don’t panic if your numbers look weird at first. That’s normal. Give it a week to see the full picture. Join online communities. Reddit’s r/CGM and Facebook groups have thousands of users sharing tips on adhesion, alarm settings, and interpreting data. You’re not alone.Final Thoughts
A continuous glucose monitor isn’t magic. But it’s the closest thing we have to a superpower for diabetes management. It turns guesswork into knowledge. It replaces fear with control. For people who live with the constant uncertainty of blood sugar swings, it doesn’t just improve health-it improves life. The numbers matter. But the trend arrows? They’re what save lives.Can CGMs replace fingerstick tests completely?
For most modern CGMs like the FreeStyle Libre 3 and Dexcom G7, yes-when used as intended. These devices are FDA-approved for non-adjunctive use, meaning you don’t need to confirm readings with a fingerstick under normal conditions. But during rapid glucose changes-like after eating, during exercise, or if you feel symptoms-you should still use a fingerstick to verify. The 5-20 minute lag between blood and interstitial fluid means CGMs can miss sudden drops or spikes.
How accurate are CGMs compared to blood glucose meters?
Modern CGMs are very accurate, with most having a MARD (mean absolute relative difference) between 7.9% and 9.1%. For comparison, a typical blood glucose meter has a MARD of about 10-15%. The Abbott FreeStyle Libre 3 is the most accurate at 7.9%, while Dexcom G7 is at 9.1%. This means, on average, a CGM reading will be within 7-9% of your actual blood sugar. That’s close enough for clinical decisions, but not perfect.
Do CGMs work during exercise or sleep?
Yes, and that’s one of their biggest advantages. CGMs track glucose 24/7, including during sleep and physical activity. Many users rely on nighttime alerts to prevent dangerous lows. During exercise, the lag between blood and interstitial fluid can cause temporary inaccuracies, so it’s wise to check with a fingerstick if you feel symptoms like shakiness or dizziness. Still, the trend arrows help you adjust carbs or insulin before your numbers crash.
How long do CGM sensors last?
Sensor lifespan varies by brand. Dexcom G7 lasts 10 days. Abbott FreeStyle Libre 3 lasts 14 days. The Eversense E3 implant lasts up to 180 days (6 months). Most users replace sensors every 7 to 14 days, depending on wear, adhesion, and accuracy. Some report getting a few extra days out of a sensor, but manufacturers don’t guarantee performance beyond the labeled duration.
Are CGMs covered by insurance?
In the U.S., most private insurers and Medicare cover CGMs for people with type 1 diabetes who use insulin. Coverage for type 2 diabetes patients varies by plan and requires proof of insulin use. Flash monitors like FreeStyle Libre were not covered by Medicare until 2023. In the UK, the NHS provides FreeStyle Libre 2 to eligible patients at no cost. In other countries, coverage depends on national health policies. Always check with your provider and ask about out-of-pocket costs before starting.
