For a 150-pound adult, the formal safety limit on aspartame alone would allow roughly 17 cans of Diet Coke per day. But caffeine, acid exposure, and longer-term health signals all tighten that number considerably. Most of the available evidence points to one or two cans a day as a reasonable amount, with risk signals starting to appear in studies of people who drink several cans daily over many years.
What the Aspartame Math Looks Like
The FDA sets the acceptable daily intake for aspartame at 50 milligrams per kilogram of body weight. A 12-ounce can of Diet Coke contains about 200 milligrams of aspartame. For someone weighing 150 pounds (68 kilograms), that works out to a daily ceiling of 3,400 milligrams, or about 17 cans. A 120-pound person would hit a limit around 13 cans. These numbers are intentionally conservative, built with a wide safety margin below the doses that caused problems in animal studies.
In 2023, the International Agency for Research on Cancer classified aspartame as “possibly carcinogenic to humans,” its third-highest category out of four. That sounds alarming, but the classification reflects how strong the evidence is that something can cause cancer, not how likely it is to cause cancer at normal intake levels. The limited evidence pointed specifically to liver cancer and came from studies that couldn’t rule out other explanations. The WHO’s joint expert committee reviewed the same data and kept the acceptable daily intake unchanged, concluding that typical consumption levels don’t warrant concern.
Caffeine Is the Tighter Limit
A 12-ounce can of Diet Coke contains roughly 46 milligrams of caffeine. The generally accepted safe ceiling for healthy adults is 400 milligrams per day. That means caffeine becomes the limiting factor before aspartame does, capping you at about eight or nine cans. At that point you’d likely notice jitteriness, disrupted sleep, or a racing heart well before you hit a clinical danger zone. If you’re also drinking coffee or tea, those cans eat into the same 400-milligram budget.
What Happens to Your Kidneys
The most striking long-term data comes from the Nurses’ Health Study, which followed more than 3,000 women for over 20 years. Women who drank several diet sodas a day experienced a 30% greater decline in kidney function compared to women who avoided diet soda entirely. In practical terms, kidney filtration normally drops by about 1 milliliter per minute per year after age 40. In the diet soda group, it dropped by 3 milliliters per minute per year, triple the expected rate.
That association held up even after the researchers adjusted for age, obesity, high blood pressure, diabetes, smoking, and overall calorie intake. Interestingly, sugar-sweetened sodas showed no similar connection. The study population was predominantly white women, so the findings may not apply equally to everyone, but it’s the best long-term evidence available on this specific question. The key detail: the accelerated decline showed up in women drinking “several” diet sodas daily, not one.
Tooth Enamel and Acid Exposure
Diet Coke contains phosphoric acid and citric acid, both of which lower the pH in your mouth and can erode tooth enamel over time. Switching from regular to diet soda saves you calories, but it does nothing to protect your teeth. The erosion is cumulative, so the more cans you drink and the longer the acid sits on your teeth, the more damage accumulates. Sipping a single can over an hour exposes your enamel to more acid than drinking it in a few minutes, so drinking pattern matters as much as volume.
Bone Density in Women
Data from the Framingham Osteoporosis Study, which looked at about 2,500 men and women between ages 49 and 69, found that cola intake was linked to lower bone mineral density at the hip in women. Non-cola carbonated drinks didn’t show the same association, and men weren’t affected. The researchers suggested caffeine might be the real culprit, since other studies have linked caffeine to lower bone density. There’s also a simpler explanation: people who drink a lot of cola tend to drink less milk, and that displacement of calcium-rich beverages may matter more than anything the soda itself does to bone.
Gut Bacteria and Blood Sugar
Animal studies on artificial sweeteners frequently show reductions in beneficial gut bacteria and increases in harmful strains, but these findings haven’t translated cleanly to humans. Human studies generally show milder changes or no significant shifts in gut microbiome composition. One exception is research on saccharin, where some participants (dubbed “responders”) developed disrupted gut bacteria and impaired glucose tolerance, while others were unaffected. The variability suggests that individual biology plays a significant role in how your gut reacts to artificial sweeteners.
On blood sugar and insulin, the clinical picture is similarly mixed. Most human studies report no significant effect or even slight benefits from artificial sweeteners on body weight and blood sugar control. However, the majority of these studies were short-term, lasting weeks to months rather than years, so they may miss slower-developing effects.
A Practical Number
Pulling together the aspartame limits, caffeine ceiling, kidney data, and dental concerns, one to two cans of Diet Coke per day falls comfortably within every known safety threshold. At three to four cans, you’re still well under the formal aspartame and caffeine limits, but you’re increasing your acid exposure and starting to approach the “several per day” range where the kidney data gets concerning over decades. Beyond four or five cans daily, the caffeine load alone reaches levels that affect sleep quality and cardiovascular comfort in most people, and long-term kidney and dental risks become harder to dismiss.
If you’re drinking Diet Coke mainly for the caffeine and flavor, keeping it to one or two cans and filling the rest of your fluid intake with water or sparkling water gives you what you’re looking for without accumulating the risks that show up at higher consumption levels.