Taking too much vitamin C can cause uncomfortable side effects and, over time, may raise the risk of kidney stones. The tolerable upper limit for adults is 2,000 mg per day, and problems tend to start around that threshold. A single large dose won’t cause lasting harm for most people, but consistently exceeding the limit is where real risks begin.
How Much Is Too Much
The National Institutes of Health sets the tolerable upper intake level for vitamin C at 2,000 mg per day for adults 19 and older, including during pregnancy and lactation. For teenagers (14 to 18), the cap is 1,800 mg. Children ages 9 to 13 should stay under 1,200 mg, those 4 to 8 under 650 mg, and toddlers 1 to 3 under 400 mg. These limits apply to vitamin C from all sources combined: food, fortified drinks, and supplements.
For context, most adults only need 75 to 90 mg of vitamin C per day. A single orange provides roughly 70 mg. It’s nearly impossible to overdose through food alone. The issue is supplements, especially the 1,000 mg tablets that are widely sold. Taking two or three of those a day puts you over the upper limit without much thought.
Your Body Can Only Absorb So Much
Your body absorbs 100% of vitamin C when you take 200 mg or less at a time. Once you go above 500 mg in a single dose, the percentage absorbed drops significantly. The higher you go, the less your body actually uses. Most of the excess passes straight through your digestive tract unabsorbed, which is exactly what triggers the most common side effect.
Digestive Problems Come First
The earliest and most noticeable symptom of too much vitamin C is diarrhea. When large amounts of unabsorbed vitamin C sit in your intestines, they pull water into the gut through osmosis, causing loose stools, cramping, bloating, and nausea. This effect kicks in around the 2,000 mg mark for most people, though some are more sensitive and notice it at lower doses. The discomfort typically stops once you cut back.
Kidney Stones Are the Bigger Concern
The more serious long-term risk is kidney stones. Your body converts a portion of vitamin C into oxalate, a waste product filtered through the kidneys. When oxalate levels in urine get too high, it can crystallize with calcium and form kidney stones.
Research published in Kidney International found that taking just 1,000 mg of supplemental vitamin C per day increased urinary oxalate by 33% to 61% in people who had already formed calcium stones. At 2,000 mg, some studies measured increases as high as 107%. Interestingly, absorption of vitamin C plateaus at very high doses, so doubling the dose doesn’t necessarily double the oxalate. But even a moderate increase in oxalate excretion matters if you’re prone to stones.
If you’ve never had a kidney stone, occasional high doses are unlikely to cause one on their own. But if you have a history of calcium oxalate stones, or a family history of them, consistently taking large vitamin C supplements adds measurable risk.
It Can Interfere With Other Nutrients
High-dose vitamin C can chemically break down vitamin B12 in your digestive tract, making the B12 inactive before your body has a chance to absorb it. If you take both supplements, spacing them at least two hours apart avoids this interaction.
Vitamin C also increases how much iron your body absorbs from food and supplements. For most people, this is neutral or even helpful. But for anyone with hemochromatosis, a genetic condition that causes iron overload, extra vitamin C can push iron levels dangerously high. The Mayo Clinic specifically advises people with hemochromatosis to avoid vitamin C supplements entirely.
It Can Skew Medical Test Results
High vitamin C levels in urine can interfere with several common lab tests. Vitamin C is an antioxidant, and it disrupts the chemical reactions that dipstick urinalysis uses to detect glucose and blood in urine. The result is false negatives: the test says everything is normal when it isn’t. The same interference can affect tests for nitrite, bilirubin, and white blood cells in urine.
This matters most if you’re being screened for diabetes, urinary tract infections, or kidney problems. If you’re taking high-dose vitamin C and have a urine test coming up, mention it to whoever ordered the test. They may ask you to stop supplementing for a day or two before re-testing, or they’ll use a different method that isn’t affected.
What a Reasonable Intake Looks Like
If you eat a few servings of fruits and vegetables daily, you’re almost certainly getting enough vitamin C without supplements. A cup of strawberries, a bell pepper, or a serving of broccoli each delivers well over 50 mg. For people who do supplement, staying at or below 500 mg per day keeps you well within the safe range, provides more than enough for immune function and tissue repair, and avoids the absorption ceiling where most of the vitamin goes to waste anyway.
If you’ve been taking 1,000 mg or more daily and feel fine, the absence of symptoms doesn’t mean there’s no effect on oxalate levels in your kidneys. The digestive symptoms are obvious, but the kidney stone risk accumulates quietly over months and years.