Vitamin C enjoys an almost mythical status in the wellness world. From fighting off colds to boosting collagen production, this essential nutrient plays crucial roles throughout the body. However, the growing trend of megadosing vitamin C supplements has nutrition experts raising red flags about potential adverse effects when consumption exceeds recommended limits.
The fine line between beneficial and harmful
While vitamin C deficiency causes scurvy, a condition virtually eliminated in developed countries, excessive supplementation creates its own set of problems. The recommended dietary allowance (RDA) for adult women is 75 milligrams daily, while men need about 90 milligrams. Pregnant women require 85 milligrams, and breastfeeding mothers need 120 milligrams daily.
However, the tolerable upper intake level—the maximum amount unlikely to cause harm—sits at 2,000 milligrams daily for adults. Many supplement enthusiasts regularly exceed this threshold, often consuming 3,000 to 10,000 milligrams daily in pursuit of perceived health benefits.
Digestive distress signals vitamin C overload
- The most immediate and common consequence of vitamin C excess manifests in the digestive system. Diarrhea frequently occurs when intake exceeds the body’s absorption capacity. This happens because unabsorbed vitamin C draws water into the intestines through osmosis, creating loose or watery stools.
For many supplement users, this digestive upset serves as the first indicator they’ve exceeded their personal tolerance threshold. The discomfort typically begins within hours of taking large doses and can persist until the excess vitamin C clears the system.
Nausea and vomiting disrupt daily activities
- Beyond diarrhea, excessive vitamin C intake often triggers nausea and vomiting in sensitive individuals. This reaction appears particularly common when taking high-dose supplements on an empty stomach.
The acidic nature of ascorbic acid can irritate the stomach lining, triggering the vomiting reflex as the body attempts to expel the irritant. For some people, even moderate supplementation can cause this reaction, while others develop symptoms only after consuming several thousand milligrams.
Heartburn and acid reflux worsen with high doses
- The relationship between vitamin C supplements and acid reflux presents particular challenges for those already managing digestive sensitivities. Ascorbic acid, as its name suggests, is indeed acidic, with a pH between 2 and 3.
This acidity can exacerbate heartburn and gastroesophageal reflux disease (GERD), especially when consumed in concentrated supplemental form rather than from food sources. People with history of ulcers or gastritis face heightened risk when taking high-dose vitamin C supplements.
Painful kidney stones develop with chronic excess
- Perhaps the most medically significant risk of excessive vitamin C consumption involves kidney stone formation. The body converts excess vitamin C to oxalate, a compound that combines with calcium to form the most common type of kidney stones.
Research published in the American Journal of Kidney Diseases found that men consuming 1,000 milligrams or more of vitamin C daily had a 41% higher risk of developing kidney stones compared to those consuming less than 90 milligrams daily. For people with history of kidney stones or kidney disease, even moderate supplementation may increase risk.
Iron overload threatens organ health
- Vitamin C dramatically enhances iron absorption from food and supplements, an effect usually considered beneficial, particularly for those with iron-deficiency anemia. However, this same mechanism creates serious risks for individuals with hemochromatosis—a condition where the body absorbs and stores excessive iron.
This genetic condition affects approximately 1 in 200 people of Northern European descent. For these individuals, high-dose vitamin C supplementation can accelerate iron accumulation in vital organs including the liver, heart, and pancreas, potentially causing permanent damage over time.
Medical test interference leads to misdiagnosis
- Excessive vitamin C intake can interfere with the accuracy of various medical tests, potentially leading to misdiagnosis or inappropriate treatment. These include:
High-dose vitamin C can cause false negative results on fecal occult blood tests, potentially masking gastrointestinal bleeding that might indicate colorectal cancer or other serious conditions. Glucose monitoring systems may show inaccurate readings in people taking large vitamin C doses. Certain urine tests for proteins and bacteria can yield misleading results when vitamin C levels in urine are elevated.
Medical professionals recommend discontinuing high-dose vitamin C supplementation at least 72 hours before scheduled laboratory tests to ensure accurate results.
Rebound scurvy presents rare but real risk
- One particularly ironic consequence of long-term high-dose vitamin C supplementation involves “rebound scurvy”—a phenomenon where the body adapts to abnormally high vitamin C levels, then experiences deficiency symptoms when intake returns to normal levels.
This occurs because the body increases production of ascorbic acid oxidase, the enzyme that breaks down vitamin C, in response to chronic high intake. When supplementation stops abruptly, this elevated enzyme activity continues, rapidly depleting the body’s vitamin C stores.
Symptoms can include fatigue, joint pain, and in extreme cases, the classic signs of scurvy such as bleeding gums and poor wound healing. While rare, this condition highlights how the body’s adaptation mechanisms can transform even beneficial nutrients into potential problems when consumed in excess.
Finding the optimal vitamin C balance
Nutrition experts emphasize that for most healthy individuals, dietary sources provide optimal vitamin C levels without risk of overdose. A single orange contains approximately 70 milligrams, while a cup of strawberries provides about 85 milligrams—already approaching the RDA for adults.
For those considering supplementation, starting with doses in the 250-500 milligram range typically provides immune support benefits without risking adverse effects. Taking supplements with food can reduce digestive discomfort, while calcium ascorbate or other buffered forms may minimize acid-related side effects.
Most importantly, remember that more isn’t always better when it comes to nutrients. The body uses what it needs and eliminates the rest—though not without potentially uncomfortable or harmful effects when overwhelmed by excessive amounts.
Before beginning any supplementation regimen, particularly at doses exceeding the RDA, consulting with a healthcare provider helps ensure the benefits outweigh potential risks for your specific health situation.
