A single can of Celsius delivers 200 mg of caffeine along with a blend of green tea extract, guarana, taurine, and ginger root, all working together to raise your metabolic rate, increase your heart rate, and sharpen your focus. That’s roughly half the FDA’s recommended daily caffeine ceiling of 400 mg for healthy adults, packed into one 12-ounce drink. Here’s what actually happens inside your body after you crack one open.
How Celsius Affects Your Heart and Blood Pressure
The most immediate effect is cardiovascular. A randomized trial published in the Journal of the American Heart Association tested energy drinks containing 304 to 320 mg of caffeine (comparable to one Celsius Essentials or a can and a half of regular Celsius) and found that systolic blood pressure rose by roughly 14 to 16 points above baseline, significantly more than a placebo drink. That spike persisted across every time point measured in the study. For context, a normal resting blood pressure reading is around 120, so a 15-point jump puts you temporarily into elevated territory even if you started perfectly healthy.
Interestingly, heart rate didn’t increase significantly more with the energy drinks than with the placebo in that same study. The blood pressure effect appears to come from the combination of caffeine and other active ingredients rather than from a simple adrenaline-like response that speeds up your pulse. If you already have high blood pressure or a heart condition, that temporary spike matters more than it does for someone with normal readings.
The Caffeine Hit: Faster and Slower at Once
Celsius gets its caffeine from two sources: synthetic caffeine and guarana seed extract, a plant-based stimulant from the Amazon. These two forms behave differently in your body. Pure caffeine absorbs rapidly, producing a sharp peak in alertness followed by a steep dropoff. Guarana contains the same caffeine molecule but also carries additional plant compounds that slow its release, creating a more gradual stimulant curve.
Modeling of guarana’s stimulant effect shows it still delivers about 50% of its peak stimulation three hours after consumption, compared to just 22% for pure caffeine at the same time point. By hour five, guarana retains about 35% of its effect while pure caffeine has dropped to around 10%. This is why many Celsius drinkers report a smoother, longer-lasting energy boost compared to coffee or other energy drinks. It’s also why drinking one in the late afternoon can interfere with sleep more than you’d expect.
What Happens to Your Metabolism
Celsius markets itself as a thermogenic drink, meaning it’s designed to increase the number of calories your body burns at rest. The mechanism behind this involves two of its key ingredients working in tandem. Green tea extract contains compounds called catechins that block an enzyme responsible for breaking down norepinephrine, a chemical your nervous system uses to signal cells to release stored energy. Caffeine, meanwhile, blocks a different enzyme that would normally dampen that same signal inside your cells.
The result is that norepinephrine’s effects last longer and hit harder than they would from caffeine alone. Your body generates slightly more heat, burns a bit more fat for fuel, and your overall metabolic rate ticks upward. This is a real, measurable effect, but it’s modest. You’re not going to burn hundreds of extra calories from a can of Celsius. The thermogenic boost is more relevant for someone who’s already exercising and eating well than for someone hoping the drink itself will drive weight loss.
Jitters, Anxiety, and Sleep Disruption
At 200 mg per can (or 270 mg for Celsius Essentials), the caffeine load is substantial. Common side effects include increased anxiety, heart palpitations, and the classic jittery feeling that comes with stimulant intake. Your individual sensitivity depends on genetics, body weight, and how much caffeine you consume regularly. Someone who rarely drinks coffee will feel 200 mg far more intensely than a habitual caffeine user.
Because guarana extends the duration of caffeine’s effects, Celsius can disrupt sleep even when consumed six or more hours before bed. Caffeine blocks your brain’s ability to recognize adenosine, the compound that builds up throughout the day and makes you feel sleepy. With guarana slowing the release, that blocking effect lingers longer than it would from a cup of coffee with the same total caffeine content.
Drinking two cans in a day puts you at 400 mg, right at the FDA’s upper limit. Two cans of Celsius Essentials would push you to 540 mg, well past that threshold and into territory where headaches, digestive upset, and rapid heartbeat become more likely.
Effects on Your Gut and Blood Sugar
Celsius is sweetened with sucralose instead of sugar, which keeps the calorie count low but introduces its own set of effects. Research published in Frontiers in Nutrition found that sucralose can influence gut bacteria composition even at the small concentrations used in beverages. The interaction between sucralose and gut microbes produces metabolites that may affect liver function and insulin signaling. Some studies in healthy young adults have shown altered insulin and glucose responses after sucralose consumption, though the long-term significance of these changes is still being studied.
The ginger root extract in Celsius may partially offset digestive discomfort, as ginger has well-documented anti-nausea properties. Still, some people report stomach upset after drinking Celsius on an empty stomach, likely from the combination of high caffeine and carbonation hitting an empty digestive tract.
How Long the Effects Last
You’ll feel the first effects within 15 to 30 minutes as caffeine enters your bloodstream. The thermogenic and alertness effects peak somewhere around 45 to 90 minutes in, then taper gradually. Thanks to the guarana component, expect noticeable stimulation for four to five hours, with residual effects stretching to six or eight hours depending on how quickly your body processes caffeine.
Caffeine’s half-life in most adults is about five hours, meaning half the caffeine from a can you drank at noon is still circulating at 5 PM. The blood pressure elevation documented in clinical research was still measurable hours after consumption, so the cardiovascular effects track closely with the stimulant timeline. Your body doesn’t simply “reset” when the energy feeling fades. The physiological effects linger after the subjective buzz wears off.
Who Should Be More Careful
Celsius is not recommended for people under 18, and the company states this on its labeling. Pregnant or nursing women should also avoid it, as caffeine crosses the placenta and enters breast milk. People with caffeine sensitivity, anxiety disorders, or cardiovascular conditions will feel the effects more acutely and face higher risks from the blood pressure spike alone.
If you’re combining Celsius with pre-workout supplements, coffee, or other caffeinated products throughout the day, it’s easy to overshoot the 400 mg daily limit without realizing it. A morning coffee (95 mg) plus a Celsius (200 mg) plus an afternoon tea (50 mg) already puts you at 345 mg before accounting for any chocolate, soda, or medication that contains caffeine.