Vitamin C is the world's most popular supplement — and the most overhyped. Our research, from 102 RCTs, 15 meta-analyses, and 67,662 participants, reveals the truth: it does NOT prevent colds (Grade D), provides only a small reduction in cold duration (Grade C), and most of its other benefits require you to actually be deficient. That said, it's a potent antioxidant, supports immune function, and enhances iron absorption. It's essential — just not the miracle cure people think it is.
Enhances drug effects
Limit to 200mg/day with deferoxamine. Avoid in heart failure patients.
Reduces drug absorption
Don't take vitamin C with ADHD stimulant medications.
May reduce effectiveness
Monitor INR if taking >1g/day vitamin C.
Increases drug absorption
Monitor thyroid levels.
Enhances iron absorption
Useful pairing for iron-deficient people. Avoid if hemochromatosis.
Increases drug half-life
Not clinically significant at normal doses.
May reduce drug effectiveness
Discuss with oncologist before supplementing during chemo.
Oral contraceptives may lower vitamin C levels, but evidence is mixed and outdated.
Not Prohibited.
Oral contraceptives may lower vitamin C levels, but evidence is mixed and outdated.
Reviewed by the Scan Dose Research Team and Clinical Advisory Board | Last updated: April 5, 2026
Not medical advice. Based on published clinical research and systematic reviews.