What are you not supposed to take with vitamin C? A comprehensive guide

January 8, 2026 •

4 min read

According to research from the National Institutes of Health, high doses of vitamin C can interfere with the absorption and effectiveness of certain nutrients and medications. This guide will detail the crucial interactions you need to know to prevent adverse health effects and maximize your wellness.

Quick Summary

Understanding which medications and supplements should not be combined with vitamin C is essential for avoiding negative interactions and health risks. Proper timing and dosage can maximize benefits safely.

Key Points

Vitamin B12: High-dose vitamin C can interfere with vitamin B12 absorption, so it is best to take supplements at least two hours apart.
Aluminum Antacids: Taking vitamin C with antacids that contain aluminum increases the body’s absorption of aluminum, posing a risk, especially for those with kidney issues.
Warfarin (Blood Thinners): High doses of vitamin C can decrease the effectiveness of warfarin, requiring careful medical supervision and blood monitoring.
Chemotherapy: The antioxidant properties of vitamin C may interfere with some chemotherapy agents; patients should not take high-dose supplements without their oncologist's approval.
Copper Supplements: Combining high doses of vitamin C and copper can induce oxidative stress and potential kidney damage; separate their intake by several hours.
Estrogen Medications: Excessive vitamin C intake can increase estrogen levels, potentially worsening side effects associated with birth control or hormone replacement therapy.
Hemochromatosis: Because vitamin C enhances iron absorption, individuals with iron overload should avoid high-dose vitamin C supplements.

Key Supplement Interactions with Vitamin C

While vitamin C is a beneficial nutrient, its high-dose supplementation can create unexpected interactions with other vitamins and minerals. Being aware of these can help you avoid potential health complications and ensure your body is absorbing the nutrients it needs.

Vitamin B12

Some studies suggest that taking high doses of vitamin C (more than 500 mg) at the same time as vitamin B12 may reduce the absorption of B12. The mechanism involves vitamin C potentially altering the B12's chemical structure during digestion, making it less bioavailable. Although not all studies have conclusively demonstrated this effect in humans, it is a recommended precaution for those taking supplements to ensure optimal absorption of both. To avoid any potential issue, experts suggest separating your vitamin C and B12 doses by at least two hours.

Copper

Mixing copper supplements with high-dose vitamin C is a significant concern due to the risk of oxidative stress and kidney damage. In the body, vitamin C can reduce copper ions (Cu2+) to a reactive form, which, in high concentrations, can promote the formation of damaging free radicals. This can overwhelm the body's antioxidant defenses. Individuals considering supplementing with copper should take it at a different time than their vitamin C supplement, preferably spaced by at least two to three hours.

Excessive Iron

While vitamin C is famously known to enhance the absorption of non-heme iron (from plant sources), this can be problematic for some individuals. For most people, this interaction is beneficial. However, for those with conditions that cause excessive iron accumulation, such as hemochromatosis, high-dose vitamin C can exacerbate the issue. The resulting iron overload can lead to serious organ damage. It is vital for individuals with these conditions to discuss their iron and vitamin C intake with a healthcare provider.

Medications that Interact with Vitamin C

It is crucial to understand how vitamin C can affect the action of prescription and over-the-counter medications. The impact can range from reducing a drug's efficacy to increasing the risk of side effects.

Aluminum-Containing Antacids

Antacids containing aluminum, such as certain brands of Mylanta and Gaviscon, should not be taken close to vitamin C supplements. Vitamin C increases the amount of aluminum your body absorbs from these medications. This heightened absorption can lead to high aluminum levels in the body, which can be especially harmful for people with kidney problems. To minimize risk, take vitamin C supplements at least two hours before or four hours after consuming aluminum-based antacids.

Chemotherapy

This is one of the most critical interactions to discuss with an oncology team. Some cancer treatments rely on creating oxidative stress to kill cancer cells, while vitamin C acts as a powerful antioxidant. The concern is that high doses of vitamin C could theoretically protect cancer cells from the oxidative damage that the chemotherapy is designed to inflict, potentially making the treatment less effective. While the evidence is not entirely consistent, the risk is significant enough that patients are often advised to avoid high-dose vitamin C supplements during treatment. Oral intake from a healthy diet is generally considered safe and beneficial.

Warfarin (Blood Thinners)

Warfarin is a medication that slows blood clotting. High doses of vitamin C have been reported to decrease the effectiveness of warfarin, increasing the risk of blood clots. Patients on blood thinners must maintain consistent nutrient intake to ensure their medication remains effective. Any high-dose vitamin C supplementation should be discussed with a doctor, and blood clotting levels should be monitored regularly.

Estrogen-Containing Medications

High doses of vitamin C can increase estrogen levels in the body. This interaction can occur with hormonal birth control pills or hormone replacement therapy. While it does not typically reduce the contraceptive's effectiveness, elevated estrogen levels can lead to increased side effects such as nausea, headaches, and mood swings. Women taking these medications, especially if they are also taking high-dose vitamin C supplements (over 1000 mg), should discuss this with their healthcare provider.

NSAIDs (Non-Steroidal Anti-Inflammatory Drugs)

Combining high doses of vitamin C with NSAIDs like ibuprofen (Advil) and naproxen (Aleve) may increase the risk of certain kidney problems. Both NSAIDs and large amounts of vitamin C can affect kidney function. Long-term use of both together may require caution. Additionally, chronic NSAID use might increase the body's vitamin C excretion, making it harder to maintain adequate levels.

Comparison of Key Vitamin C Interactions


Interaction	Effect of Interaction	Precautionary Action
High-Dose Vitamin C & B12	May reduce B12 absorption.	Take B12 and vitamin C at least 2 hours apart.
Vitamin C & Aluminum Antacids	Increases aluminum absorption, risky for kidneys.	Take 2-4 hours apart; check with a doctor if you have kidney issues.
High-Dose Vitamin C & Warfarin	Can decrease the blood-thinning effect of warfarin.	Consult a doctor and monitor blood regularly if supplementing.
High-Dose Vitamin C & Estrogen	May increase estrogen levels, potentially worsening side effects.	Stick to recommended doses; space out intake with high doses.

Conclusion

While vitamin C is an essential nutrient with many health benefits, it is not a standalone supplement without potential for interaction. The key takeaway is that both dosage and timing matter greatly, especially when combining vitamin C with other supplements or medications. Many interactions, particularly those with aluminum-containing antacids and certain cancer therapies, can lead to serious health complications and should not be taken lightly. Always consult with a qualified healthcare professional before starting any new supplement regimen, especially if you have pre-existing health conditions like hemochromatosis or kidney disease, or are undergoing treatment for cancer. For more comprehensive information on supplement interactions, refer to authoritative sources such as MedlinePlus. By being informed and proactive, you can safely navigate your nutritional choices and avoid adverse effects.

MedlinePlus: Vitamin C Interactions

Frequently Asked Questions

Yes, multivitamins are typically formulated with balanced doses, so the vitamin C content is unlikely to significantly interfere with B12 absorption. However, if you are taking high-dose separate supplements, spacing them out is a safer option.

It is recommended to take vitamin C at least two hours before or four hours after taking an aluminum-containing antacid to avoid increased aluminum absorption.

No, normal dietary intake of vitamin C from foods is generally not a concern for people on blood thinners. The interactions are primarily linked to high doses of supplemental vitamin C.

For skincare, it is generally recommended to separate application of vitamin C and copper peptides by 10-15 minutes or apply them at different times of the day (e.g., vitamin C in the morning, copper peptides at night) to prevent them from potentially neutralizing each other.

Some common over-the-counter antacids that may contain aluminum include Mylanta and Gaviscon. It is always best to read the ingredients label or ask a pharmacist if you are unsure.

Normal dietary intake of vitamin C from sources like oranges is generally not a concern during chemotherapy. The potential interaction involves very high doses of concentrated vitamin C from supplements, not from food.

High doses of vitamin C (1000 mg or more) can potentially increase estrogen levels, which may lead to more pronounced birth control side effects, though it is not believed to reduce contraceptive efficacy. Normal doses are not an issue.

For occasional use, there is likely no significant risk. However, long-term use of both in high doses could potentially increase the risk of kidney problems, so chronic users should consult a healthcare provider.