Is Carbon Dioxide in Soda Harmful?

Carbon dioxide, the gas that gives soda its fizz, is not harmful to most people in the amounts found in carbonated drinks. The CO₂ dissolved in a can of soda creates carbonic acid, but it is a weak acid that your body neutralizes quickly, and a systematic review found no evidence that carbonated beverages directly damage the esophagus or lead to serious gastrointestinal complications. That said, the picture gets more complicated once you look beyond the gas itself and consider how carbonation interacts with your teeth, your appetite, your reflux symptoms, and whatever else is in the drink.

What Happens When You Swallow Carbonated Liquid

When CO₂ dissolves in water, it forms carbonic acid, which is why sparkling water has a slightly acidic pH (usually around 3 to 4). That sounds alarming if you compare it to your stomach’s normal pH of 1.5 to 3.5, but it is mild by comparison. Once the drink hits your stomach, the CO₂ rapidly comes out of solution as gas. This causes the familiar bloated or full feeling, and some of that gas comes back up as a belch.

Despite the extra gas, carbonation does not meaningfully slow or speed up how fast your stomach empties. A study of carbonated versus still water during exercise found no effect on gastric emptying rate.¹ Another study confirmed that overall emptying of both solid and liquid food was the same with carbonated or still water, though carbonation did change where the food sat inside the stomach, pushing more of the meal into the upper portion.² That redistribution is temporary and resolves as the gas dissipates, but it helps explain why a fizzy drink can make you feel fuller in the moment even though your digestion proceeds at the same pace.

Carbonation and Acid Reflux

If you already deal with heartburn or gastroesophageal reflux, you have probably heard conflicting advice about carbonated drinks. The concern is reasonable on its face: CO₂ gas could increase pressure in the stomach and force acidic contents back up into the esophagus. And there is a real physiological mechanism at work. In healthy volunteers, drinking a carbonated beverage roughly halved resting pressure at the lower esophageal sphincter, the muscular ring that keeps stomach contents from traveling upward, and also increased the frequency of spontaneous sphincter relaxations compared to drinking plain water.³

Yet when researchers looked at the bigger picture across multiple studies, the clinical significance turned out to be modest. A systematic review concluded that while carbonated beverages briefly lower esophageal pH and can transiently reduce sphincter pressure, there is no evidence they directly cause esophageal damage, and they have not been consistently shown to produce reflux-related symptoms.⁴ The same review found no link between carbonated drinks and esophageal cancer or other serious complications. So if you have occasional reflux, carbonation might make a particular episode feel worse, but it does not appear to cause lasting harm. People with severe or chronic reflux may still want to limit fizzy drinks as a comfort measure, but the gas itself is not eroding your esophagus.

What Carbonation Does to Your Teeth

Dental erosion is where the carbonation question gets more nuanced. Plain carbonated water is acidic enough to slightly soften tooth enamel under laboratory conditions, and higher levels of carbonation cause more surface hardness loss than lower levels.⁵ In that same lab study, adding calcium ions to the carbonated water reduced the effect, which hints at why mineral-rich sparkling waters may be less of a concern than homemade seltzer from a soda machine.

Context matters here. Lab studies soak enamel samples in carbonated water for extended periods that do not reflect how most people drink. In real life, saliva quickly remineralizes the enamel surface after you take a sip. A randomized crossover trial in adolescents found that salivary pH stayed above 7.0 after drinking plain carbonated water, with no significant pH drop, whereas a sugar-sweetened soda pulled salivary pH down to around 6.8 and kept it suppressed for roughly five minutes.⁶ The practical takeaway: plain sparkling water is far gentler on your teeth than sugary soda, and even within sugary sodas, the sugar and citric acid additives are doing most of the dental damage, not the CO₂ itself.

Oral Bacteria and Sugar, Not Just the Bubbles

The same adolescent trial measured bacterial growth in saliva after each drink, and the results highlight how much the non-CO₂ ingredients matter. Sucrose-sweetened soda produced a massive spike in oral bacteria, artificially sweetened soda produced a smaller but still significant increase, and plain carbonated water produced the smallest rise of the three.⁶ All three were statistically higher than plain water, so carbonation is not entirely innocent, but the sugar-driven bacterial growth dwarfed the carbonation-only effect by roughly five to one. If you are trying to protect your teeth, switching from regular soda to sparkling water eliminates most of the risk. Sipping through a straw and rinsing with plain water afterward handles much of the rest.

Does Carbonation Affect Appetite and Weight

One widely shared concern is that carbonated drinks might make you hungrier. This stems from a study in rats and a small parallel human experiment, both of which found that drinking carbonated beverages increased blood levels of ghrelin, the so-called hunger hormone.⁷ The rats given carbonated drinks ate more and gained more weight than those given flat versions of the same drinks, and 20 healthy men showed higher ghrelin levels after carbonated drinks compared to controls.

The finding is interesting but needs to be kept in proportion. The human arm of that study was small and measured a hormone level, not actual long-term eating behavior or weight gain. Ghrelin rises and falls throughout the day for many reasons, and a transient spike does not automatically translate into eating more at your next meal. No large-scale human trials have confirmed that switching from still to carbonated water causes meaningful weight gain. In fact, the short-term bloating and fullness from carbonation may actually suppress appetite in the moment for some people. This is an area where the science is thin and the headlines have outrun the evidence.

Bone Health and the Cola Confusion

The idea that carbonated drinks weaken your bones has been around for decades, but the evidence points at cola specifically, not carbonation in general. A study of adolescents in Northern Ireland found that non-cola carbonated drink consumption was associated with lower heel bone mineral density in girls, and diet drinks showed a similar relationship, but cola and non-diet drinks were not consistently linked to reduced bone density in either sex.⁸ That pattern is hard to explain by carbonation alone, since all the drinks were fizzy.

The leading hypothesis for any bone-density link involves phosphoric acid, which is found in colas but not in most other carbonated beverages, and the possibility that heavy soda drinkers are simply drinking less milk. When you control for calcium intake, the association between carbonated drinks and bone loss weakens or disappears. Plain sparkling water, which contains CO₂ but no phosphoric acid and no sugar, has not been linked to bone-density problems in any published research. If your concern is bone health, the issue is what is in the soda, not that it fizzes.

Your Body’s Acid-Base Balance

A common worry is that drinking something acidic will “acidify” your blood. Your body’s pH regulation is far more powerful than a can of soda. Blood pH is tightly maintained between about 7.35 and 7.45 by your lungs and kidneys, and normal dietary acids barely register against that buffering system. In fact, bicarbonate-rich mineral water, which is naturally carbonated, has been shown to increase blood bicarbonate levels and urine pH, effectively enhancing your body’s acid-buffering capacity rather than overwhelming it.⁹ So the carbonic acid in a sparkling water is not pulling your blood toward acidity. Your kidneys handle it effortlessly.

Carbonation Can Speed Up Alcohol Absorption

Here is one area where CO₂ does have a real practical effect that most people do not know about. A controlled study of 21 subjects found that drinking alcohol with a carbonated mixer led to faster alcohol absorption in two-thirds of participants, with the average absorption rate significantly higher than when the same alcohol was mixed with a still drink.¹⁰ The effect varied between individuals — about a third showed no change or a slight decrease — but the overall trend was clear. This is worth knowing if you are pacing yourself on a night out: a vodka tonic or champagne may hit your bloodstream faster than the same amount of alcohol in a non-fizzy cocktail.

A similar phenomenon extends to at least one common medication. Research on paracetamol (acetaminophen) found that taking it with sparkling water produced a trend toward faster and more consistent absorption compared to still water, likely because the carbonation promoted quicker tablet breakdown in the stomach.¹¹ For a headache tablet you want to work quickly, that might actually be a plus. For medications where precise timing matters, it is worth being aware of the interaction.

Why Carbonation Stings

The sharp, slightly prickly sensation you feel when you drink something carbonated is not just the bubbles popping on your tongue. CO₂ activates a specific pain-sensing channel called TRPA1 on trigeminal nerve fibers in your mouth and throat, the same type of channel triggered by mustard oil and cinnamon.¹² The CO₂ dissolves into the thin film of moisture on your oral tissues and generates a brief burst of acidity right at the cell surface, which flips the TRPA1 channel open and sends a mild pain signal. This is why flat soda tastes different from freshly opened soda even though the sugar content is the same. That stinging sensation is technically a low-grade pain signal, which the brain interprets as pleasurable in the same way people enjoy spicy food.

Carbonation as a Therapeutic Tool for Swallowing Difficulty

One of the more surprising findings in the research is that carbonation can actually help people who have trouble swallowing. Older adults with oropharyngeal dysphagia, a condition where the swallowing reflex is slow or poorly coordinated, showed improved swallowing when given thickened carbonated drinks compared to non-carbonated versions. Carbonated thickened cola initiated the swallowing reflex significantly earlier than its flat counterpart.¹³ Similar results appeared in a study using carbonated thickened liquids evaluated with endoscopic imaging.¹⁴

The mechanism appears to involve sensory stimulation of the pharynx. Research using brain imaging and electrophysiology found that carbonated liquids improved the timing of swallows and increased the excitability of the brain-to-throat nerve pathway compared to still water, with the effects lasting up to an hour after the carbonated liquid was consumed.¹⁵ This is a case where the very property of CO₂ that causes the slight sting in your mouth, its activation of sensory nerve channels, becomes genuinely useful. Carbonated drinks are now sometimes used in clinical rehabilitation for people recovering from stroke or living with neurological conditions that impair swallowing.

Separating CO₂ From Everything Else in the Can

Much of the confusion around whether soda is harmful comes from conflating the effects of carbon dioxide with the effects of sugar, phosphoric acid, citric acid, caffeine, and artificial sweeteners. When researchers strip away those other ingredients and test plain carbonated water, most of the scary-sounding associations evaporate. Sugar-sweetened sodas are linked to obesity, type 2 diabetes, and cardiovascular disease in large epidemiological studies, but those outcomes are driven by the sugar and caloric load, not the fizz. Similarly, the dental erosion that dentists see in heavy soda drinkers comes overwhelmingly from the phosphoric and citric acids added as flavorings, with the carbonic acid from CO₂ playing a minor supporting role at most.

This distinction matters when you are deciding what to drink. Plain sparkling water and sugar-free seltzer are, by the available evidence, essentially as safe as still water for the vast majority of people. If you enjoy the fizz, there is no strong scientific reason to give it up. The legitimate health concerns around “soda” almost always trace back to the other ingredients in the can, not to the carbon dioxide that makes it bubble.

Who Might Want to Limit Carbonation

While CO₂ in drinks is safe for most people, a few groups have reasonable grounds for cutting back. People with irritable bowel syndrome or chronic bloating sometimes find that the extra gas from carbonation worsens distension and discomfort, even if it does not cause any structural damage. Those with severe gastroesophageal reflux may find that the transient drop in sphincter pressure from carbonation makes individual episodes more frequent or uncomfortable, even though the research does not show long-term esophageal harm. And anyone who notices that sparkling water seems to increase their appetite, consistent with the ghrelin-stimulating effect observed in a small study, might want to pay attention to whether they eat more after drinking fizzy versus flat beverages.

People who use a home soda maker should also be aware that heavily carbonated water, made by holding the carbonation button longer than the instructions suggest, produces a more acidic solution with a greater potential for enamel softening than commercially carbonated water at standard carbonation levels.⁵ If you crank your soda maker to maximum fizz and sip the result throughout the day, you are bathing your teeth in a somewhat more acidic environment than if you drink a normally carbonated mineral water at meals. Even then, the risk is modest compared to sugary or citrus-flavored sodas, but it is the one scenario where plain carbonated water deserves a note of caution for dental health.