Continuous Glucose Monitors: How CGMs Work and Who Benefits Most

Imagine knowing your blood sugar is dropping while you’re asleep - and getting an alert before it becomes dangerous. That’s not science fiction. It’s what continuous glucose monitors do every day for millions of people with diabetes.

How CGMs Actually Work

A continuous glucose monitor (CGM) doesn’t measure blood sugar directly. Instead, it reads glucose levels in the fluid between your cells - called interstitial fluid - using a tiny sensor inserted just under the skin. Most people wear it on their belly or upper arm. The sensor is no bigger than a coin and stays in place for days or even months, depending on the model.

Inside the sensor is a special enzyme that reacts with glucose. When glucose touches it, a small electrical signal is created. That signal gets sent wirelessly to a receiver or your smartphone. Some devices, like the Dexcom G7, send updates every 5 minutes. Others, like the FreeStyle Libre 3, update every single minute. You can see your current number, but more importantly, you see where your glucose is going - up, down, or steady - thanks to trend arrows.

There’s one catch: there’s a 5 to 20-minute delay between what’s happening in your blood and what the CGM shows. That’s because glucose moves from blood into interstitial fluid slowly. So if your sugar drops fast during a workout, the CGM might lag behind. That’s why doctors still recommend checking 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 are the same. There are two main types: real-time and flash.

Real-time CGMs - like Dexcom G7 and Eversense E3 - send data automatically. They beep or vibrate if your glucose goes too high or too low, even while you’re sleeping. You don’t have to do anything. The system just works.

Flash monitors - like the FreeStyle Libre 3 - need you to scan the sensor with your phone to see your numbers. No automatic alerts. But they also don’t require fingerstick confirmations, and they’re cheaper. Many users like them because they’re less intrusive. You scan when you want to check, not because the device is nagging you.

Here’s how the top models stack up:

FreeStyle Libre 3 is the most affordable and easiest to use. Dexcom G7 is the most accurate but costs nearly four times as much. Eversense E3 lasts six months but requires a small procedure to insert and remove it. Your choice depends on your lifestyle, budget, and how much you want the device to manage your diabetes for you.

Who Benefits the Most From a CGM?

If you have type 1 diabetes, a CGM isn’t just helpful - it’s life-saving. Studies show people using CGMs reduce their risk of dangerous low blood sugar episodes by 31%. That’s huge. One Reddit user, u/Type1Since1995, said his Libre 3 alarm saved him from a blood sugar of 32 mg/dL at 3 a.m. - he’d have been unconscious without it.

People with type 2 diabetes who use insulin multiple times a day also benefit significantly. A 2023 American Diabetes Association report found these users lowered their A1C by 0.5% to 0.8% - enough to reduce long-term complications like nerve damage, kidney problems, and vision loss.

Even non-diabetics are starting to use CGMs. Athletes, biohackers, and people trying to manage energy crashes or weight gain are wearing them to see how food affects their glucose. But here’s the catch: insurance won’t cover CGMs for non-diabetics. They’re out-of-pocket only, and experts warn the data can be misleading without medical context.

Older adults with multiple health conditions might find CGMs overwhelming. Too many alerts can cause anxiety, not relief. One doctor at the University of North Carolina warned that for some, the constant stream of numbers creates more stress than control.

What Are the Downsides?

CGMs aren’t perfect. The biggest complaint? Skin irritation. The adhesive on Dexcom sensors can cause rashes, especially if you sweat a lot or have sensitive skin. Many users swear by Skin Tac wipes or medical tape like Opsite Flexifix to keep sensors stuck.

Another issue? False alarms. One user on a diabetes forum said his Dexcom showed a low during a bike ride - but his fingerstick read normal. He ate carbs unnecessarily, then crashed later. That’s the lag problem again. If your glucose is changing fast, the CGM can’t keep up.

And then there’s cost. Without insurance, CGMs can run $300-$400 a month. That’s not affordable for everyone. But in the UK, the NHS gives FreeStyle Libre 2 to over half a million people for free. In the US, Medicare now covers CGMs for insulin users, but private insurers vary widely. Always check with your provider.

How to Get Started

Using a CGM isn’t hard, but it does take a little learning. First, you need a prescription. Your doctor will decide if you qualify based on your diabetes type, insulin use, and history of low blood sugar.

Once you have it, setup takes about 15-20 minutes. Insert the sensor, pair it with your phone, and wait for it to warm up. Dexcom G7 now starts in 30 minutes - down from two hours just a year ago. FreeStyle Libre 3 works immediately after scanning.

The real challenge? Learning what the numbers mean. Trend arrows are your best friend. An arrow pointing down fast means you need to eat. One pointing up means you might need insulin. Ignore the number for a second - look at the direction.

Most manufacturers offer free apps with tutorials. Dexcom Clarity and LibreView show daily patterns so you can see which meals spike your sugar or which workouts help lower it. Online communities like the 45,000-member CGM Users Facebook group are full of people sharing tips on adhesion, alarm settings, and how to avoid burnout.

What’s Next for CGMs?

The future is getting even smarter. Abbott just launched FreeStyle Libre 3 Plus, which tracks insulin doses right in the app. Tandem’s Control-IQ system, coming in early 2024, can predict a low 30 minutes before it happens and automatically pause insulin delivery.

Apple is rumored to be developing a glucose-sensing smartwatch - if it happens, it could bring CGM tech to millions who never thought they’d need it. Non-invasive sensors, like Glucowise, are in late-stage testing. These would measure glucose through the skin without any needle.

But the biggest win? Fewer hospital visits. A 2023 study in the New England Journal of Medicine found CGM users had 24% fewer diabetes-related hospitalizations. That’s not just better health - it’s less stress, fewer missed workdays, and more peace of mind.

Final Thoughts

A CGM isn’t a magic cure. But for people managing diabetes, it’s the closest thing to a superpower. It gives you control when you need it most - at night, during exercise, after meals - without poking your finger every few hours.

If you’re on insulin, have frequent lows, or just want to understand how your body responds to food and activity, a CGM can change everything. Talk to your doctor. Check your insurance. Try one. You might find, like thousands of others, that you never want to go back to fingersticks again.