Thermogenic (or ‘fat-burner’) supplements are all over the place—at the gym, in your social media feed, and spread far and wide across The Vitamin Shoppe shelves. But whether you’ve tried them already or are simply considering it, how much do you really know about how thermogenics work?

Let’s Talk About Thermogenesis

First, let’s back up a few steps. Your metabolism is the amount of energy your body needs for various functions, from breathing to digesting food to contracting your muscles. The more muscle mass you have and the more you exercise—among other things—the more energy (a.k.a. calories) your body burns through every day, according to the National Institutes of Health.

Whenever our body needs to produce or use energy, the chemical reactions that occur create heat as a by-product, says Brian Tanzer, M.S., nutritionist and manager of scientific affairs for The Vitamin Shoppe. The resulting increase of heat in our bodies is called thermogenesis. We measure thermogenesis—and how much ‘burned’ energy it represent—in calories.

Here are the Spark notes: Thermogenesis boosts our metabolism—which means we burn more calories. And burning more calories is, of course, a major factor in maintaining a healthy body weight or shedding body fat.

How Thermogenic Supplements Work

Thermogenesis occurs to some degree pretty much whenever we need energy, but the shtick of thermogenic supplements is that they boost that thermogenesis—and our calorie-burning—even further, says Tanzer.

But, uh, how does a supplement magically burn calories? Most thermogenic supps contain stimulants, ingredients that speed up the function of certain parts of your body, boosting your heart rate and breathing rate, according to the U.S. National Library of Medicine. Because these systems are chugging at a quicker rate, your body burns through more energy, says Tanzer.

Some stimulants also increase your production of certain stress hormones, like epinephrine (a.k.a. adrenaline) sending a ‘fight or flight’ signal to your body. This increases blood pressure and boosts your ability to tap into fat stores for fuel, according to the University of Delaware.

Related: 7 Natural Ways To Kickstart Your Metabolism

The most well-known stimulant is something many of us already have a daily relationship with: caffeine. Caffeine does all of the stuff we just mentioned, and is found in most thermogenic supps, often along with other ingredients that relate to metabolism in some way, says Tanzer. Some of these other common ingredients include: yohimbe, green tea extract (EGCG), capsicum extract, and B vitamins.

First up is yohimbe, a compound found in the bark of an African tree. “Yohimbe works somewhat similar to caffeine, and can increase your production of hormones involved with fat metabolism,” Tanzer says. But since these hormones are part of your body’s stress response, they can make some people feel on edge, he adds.

EGCG (epigallocatechin gallate) is one of the most abundant polyphenols found in green tea, says Tanzer. (Polyphenols are plant compounds that have antioxidant properties.) And, according to a review published in the Journal of the American College of Nutrition, EGCG has been shown—often in combination with caffeine—to slightly boost energy expenditure (calorie-burning) and fat oxidation (fat burning) over a 24-hour period. How does EGCG work? By preventing the breakdown of the hormone epinephrine and keeping your body in high-alert stress-response mode, Tanzer says.

Related: How I Kicked My Coffee Habit For Tea—And Lived Happily Ever After

‘Capsicum extract’ is just the science-y way of saying ‘hot pepper extract.’ This one works by just straight-up producing heat in the body without stimulating the stress hormones that tell your heart rate and breathing rate to kick up, Tanzer explains. “Your body wants to get back to homeostasis [a.k.a. its normal temperature] and burns calories to get there,” he says.

And then there are B vitamins. While B vitamins don’t directly stimulate your body, they enable your body to convert food into energy, Tanzer says. They may not be the lead actor, but they play an important supporting role in your body’s energy breakdown and creation process.

Are Thermogenics Right for You?

Anyone who’s had one too many cups of coffee knows, you can have too much of a good thing. Stimulants can make some people feel restless, irritable, or jittery, according to The Mayo Clinic. And when you take a number of stimulants together, it’s hard to know how that combination will affect you, says Tanzer.

Considering that, anyone with high blood pressure, cardiovascular issues, prone to seizures, on psychiatric medications, or with a family history of heart issues or stroke, may run into trouble with these supplements, Tanzer explains.

If you check with your doctor and get the go-ahead to try a thermogenic supplement, follow the instructions on the label very carefully, Tanzer says. (Most labels will tell you to test your reaction to the supp first by taking just a small dose.) If you notice any side effects—at any point—it’s time to breakup with your fat-burning friend.