Continuous Glucose Monitors: How CGMs Work and Who Benefits

Imagine knowing your blood sugar is dropping while you’re asleep-before you even feel dizzy or sweaty. That’s not science fiction. It’s what a continuous glucose monitor does for millions of people with diabetes every day. Unlike fingerstick tests that give you a single snapshot, CGMs track your glucose levels 24/7, showing not just the number but whether it’s rising, falling, or staying steady. This isn’t just convenient-it’s life-saving.

How Does a CGM Actually Work?

A CGM doesn’t measure blood sugar directly. Instead, it reads glucose in the fluid between your cells-the interstitial fluid-through a tiny sensor inserted just under your skin. Most people wear it on their belly or upper arm. The sensor has a thin filament coated with an enzyme that reacts with glucose. When glucose interacts with it, it creates a small electric current. That current gets converted into a glucose reading.

The sensor sends that data wirelessly to a receiver or your smartphone. Modern devices like the Dexcom G7 and Abbott FreeStyle Libre 3 update every 1 to 5 minutes. That means you’re getting up to 288 readings a day, not just 4 or 5 like with a traditional meter.

There’s one catch: interstitial fluid glucose lags behind blood glucose by 5 to 20 minutes. That’s why, during a fast spike after eating or a sudden drop during exercise, your CGM might not match your fingerstick exactly. It’s not wrong-it’s just slightly behind. That’s why experts say: always check with a fingerstick if you feel symptoms but the CGM doesn’t match how you feel.

Real-Time vs. Flash Monitoring: What’s the Difference?

Not all CGMs work the same way. There are two main types: real-time and flash.

Real-time CGMs like Dexcom G7 and Eversense E3 send data automatically. They buzz, beep, or send phone alerts when your glucose goes too high or too low-even while you’re sleeping. These are ideal for people who need constant alerts, especially those with hypoglycemia unawareness.

Flash monitors like the FreeStyle Libre 3 don’t send alerts unless you scan them. You wave your phone or reader over the sensor to see your current number and trend. No alarms. No automatic uploads. But they’re cheaper, simpler, and don’t require fingerstick confirmations anymore.

Here’s the trade-off: real-time gives you peace of mind with alerts. Flash gives you data without the noise. Which one you pick depends on your lifestyle, your risk of lows, and whether you want to be constantly notified.

Who Benefits Most From a CGM?

CGMs aren’t just for people with type 1 diabetes anymore. The American Diabetes Association now recommends them for:

• Anyone with type 1 diabetes, no exceptions
• Type 2 diabetes patients using multiple daily insulin injections
• People with frequent low blood sugar episodes, even if they don’t have diabetes
• Pregnant women with gestational or pre-existing diabetes

Why? Because CGMs cut severe hypoglycemia by 31%, according to a 2022 analysis of 32 clinical trials. They also lower A1C by 0.5% to 0.8% on average-enough to reduce long-term complications like nerve damage, kidney disease, and vision loss.

But the biggest win? Sleep. People using CGMs report fewer nighttime lows. One Reddit user wrote: “My Libre 3 alarm saved me from a 32 mg/dL hypo at 3 AM last week-I’d have been dead without it.” That’s not an exaggeration. Many people don’t wake up when their blood sugar crashes. A CGM does.

Accuracy and Limitations: What You Can’t Rely On

CGMs are accurate-really accurate. The FreeStyle Libre 3 has a mean error of just 7.9% compared to lab tests. Dexcom G7 is at 9.1%. That’s better than most fingerstick meters.

But they’re not perfect. Here are the real limitations:

• Lag time: During exercise, after eating, or after insulin, your CGM might be off by 15-20 minutes. Don’t treat a low based on the CGM alone if you’re feeling symptoms.
• Medication interference: Acetaminophen (Tylenol) can cause falsely high readings on most CGMs. Check your label or ask your pharmacist.
• Adhesion issues: Sensors fall off. Especially if you sweat a lot, shower often, or have oily skin. Skin Tac wipes or Opsite Flexifix tape help.
• False alarms: Some users report alarms during workouts when glucose drops quickly. That’s the lag again. Learn to read the trend arrow.

The trend arrow is your best friend. If it’s pointing down sharply, you’re dropping fast-even if the number is still above 70. That’s when you act.

Cost, Insurance, and Accessibility

Cost is the biggest barrier. In the U.S., Dexcom G7 runs about $399 a month without insurance. FreeStyle Libre 3 is $110 for three sensors. That’s $330 a month. Ouch.

But here’s the good news: insurance coverage is improving. Medicare now covers CGMs for insulin users. Many private insurers follow suit. In the UK, the NHS gives FreeStyle Libre 2 to half a million people for free. In Australia, the PBS subsidizes CGMs for eligible type 1 patients.

If you’re paying out of pocket, look into manufacturer programs. Dexcom and Abbott offer savings cards, payment plans, and even free sensors for new users. Some employers and health apps also offer reimbursement.

What’s Next for CGMs?

The tech is moving fast. Dexcom G7 cut its warm-up time from two hours to 30 minutes. Libre 3 Plus now tracks insulin doses right on the app. Eversense E3 lasts six months-but requires a minor procedure to insert and remove.

Next up? Non-invasive sensors. Glucowise is testing a wristband that reads glucose through infrared light-no needles. Apple is rumored to be working on a glucose-sensing watch. And hybrid closed-loop systems like Tandem’s Control-IQ are already predicting lows 30 minutes before they happen and adjusting insulin automatically.

These aren’t distant dreams. They’re here. And they’re getting better every year.

Getting Started: What You Need to Know

If you’re thinking about a CGM, here’s how to start:

1. Ask your doctor if you’re a candidate. You don’t need a prescription for Libre 3 in some countries, but you still need a prescription for insurance coverage.
2. Check your insurance. Call your provider and ask: “Do you cover continuous glucose monitors for my diagnosis?”
3. Compare devices. Do you need alerts? Or just data? Do you want to scan or get automatic updates?
4. Learn the trend arrows. Spend a week just watching how your numbers move after meals, exercise, and stress. Don’t react immediately-observe.
5. Use the apps. Dexcom Clarity and LibreView turn data into easy-to-read reports. Share them with your doctor.

Most people get comfortable in 3 to 5 days. The biggest mistake? Trying to fix every little fluctuation. CGMs show patterns, not emergencies. Let the data guide you-not panic.

Real People, Real Results

Over 12,000 users on Reddit and diabetes forums gave CGMs an average 4.2 out of 5 rating. The top praise? Nighttime safety. The top complaint? Cost. And skin irritation.

One user said: “I used to wake up every night with a panic attack because I didn’t know if I was low. Now I sleep through the night. My CGM tells me if I need to eat.”

Another: “I thought I’d hate wearing a device all the time. Turns out, I hate not knowing what my sugar is doing more.”

These aren’t just gadgets. They’re tools that restore control, reduce fear, and give people back their lives.