7 Ways Vitamin C Interacts with Your Meds

Vitamin C is a well-known nutrient that’s found in a variety of fruits and vegetables. It plays a role in many body functions, including collagen production, wound healing, iron absorption and immune health, just to name a few. Because it’s water-soluble, excess amounts of vitamin C are excreted in urine, rather than stored in the body.

Whether you’re hoping to avoid catching a cold or simply trying to boost your intake, you may have considered buying a vitamin C supplement. However, before you add it to your shopping cart, it’s important to know that vitamin C in supplement form isn’t necessary or safe for everyone. “Certain medications can interact with vitamin C supplements, potentially affecting their effectiveness or causing adverse side effects,” says Michelle Routhenstein, M.S., RD, CDCES, CDN, a registered dietitian.

- Advertisement -

We spoke with Routhenstein and two other health experts—Jennifer Bourgeois, Pharm.D., a clinical pharmacist and Edmond Hakimi, O.D., an osteopathic doctor—to understand who shouldn’t take vitamin C supplements and how this essential nutrient can interact with medications you may be taking.

1. Aluminum-Containing Drugs

Because vitamin C can increase aluminum absorption, taking vitamin C with aluminum-containing medications may lead to aluminum toxicity and impaired kidney function. Aluminum hydroxide-containing antacids, like Mylanta, Maalox and Gaviscon, and phosphate binders, which are often prescribed for people with kidney disease, are examples of aluminum-containing drugs. 

2. Blood Thinners

Vitamin C can also be dangerous for people taking blood thinners, like warfarin. When combined, high doses of vitamin C can interfere with these medications’ anticoagulant effects and increase the risk of blood clots.

3. Antacids and Proton Pump Inhibitors (PPI)

Stomach acid plays a key role in vitamin C absorption. Because antacids and PPIs, like omeprazole and lansoprazole, reduce stomach acid, taking these medications can impair your body’s ability to absorb vitamin C.

4. Nonsteroidal Anti-Inflammatory Drugs (NSAIDs) 

Aspirin, ibuprofen (Advil and Motrin) and naproxen (Aleve) are common over-the-counter NSAIDs used to reduce pain or inflammation. According to Hakimi, aspirin and other NSAIDs can increase the amount of vitamin C lost in your urine, resulting in lower vitamin C levels. 

This interaction likely isn’t concerning for people who take Advil or aspirin occasionally. However, long-term NSAID use could make it harder to maintain adequate vitamin C levels and is worth talking to your doctor about. 

5. Diabetes Medications

Vitamin C supplements have been shown to lower fasting blood sugar, insulin and hemoglobin A1C (the 3-month blood glucose average) levels in people with type 2 diabetes. Folks taking blood sugar-lowering medications, like Metformin or insulin, should talk with their doctor before taking a vitamin C supplement. 

Bourgeois also notes that high doses of vitamin C may interfere with blood sugar readings. Vitamin C can reduce pyrroloquinoline quinone (PPA), a compound used in some glucose meter strips, potentially causing falsely elevated blood sugar level readings.

6. Chemotherapy Drugs 

Chemotherapy drugs, like doxorubicin and bortezomib, produce free radicals that destroy both healthy and cancerous cells. Because vitamin C has potent antioxidant and anti-inflammatory effects, Bourgeois explains that vitamin C supplements may make chemotherapy less effective. That said, some research suggests that intravenous vitamin C may actually help alleviate certain side effects of cancer treatment. Talk with your oncologist about whether vitamin C is safe for you.

7. Antiretroviral Drugs 

Vitamin C has been shown to reduce the effectiveness of antiretroviral drugs, like protease inhibitors (Indinavir), which are used to treat human immunodeficiency virus (HIV). Lowering the effectiveness of these drugs makes the virus more challenging to control and may increase the risk of infection.

Who Should Avoid Vitamin C Supplements

Vitamin C isn’t safe or appropriate for everyone. People with the following health conditions should talk with a healthcare provider before taking vitamin C supplements:

• People with Kidney Disease or a History of Kidney Stones: High doses of vitamin C can cause a buildup of oxalate, especially in people with kidney disease, and lead to the formation of kidney stones.
• People with Diabetes: Since vitamin C may lower your blood sugar levels, talk with your doctor if you take specific medications to lower blood sugar levels or treat hypoglycemia before taking vitamin C supplements.
• People Taking Blood Thinners: Vitamin C supplements may counteract the effects of anticoagulants, like warfarin, and increase your risk for blood clots.
• People with Hemochromatosis: Since vitamin C enhances iron absorption, high doses of vitamin C could lead to iron toxicity if you have hemochromatosis—a condition that causes the body to absorb too much iron.
• People Undergoing Certain Chemotherapy Treatments: High doses of vitamin C may impair the effectiveness of some chemotherapy treatments.  
• People Being Treated for HIV: Vitamin C may lower the effectiveness of antiretroviral medications, making treating HIV more difficult.

Side Effects of Vitamin C Supplements

Despite being water-soluble, there is such a thing as too much vitamin C. The upper limit for vitamin C is 2,000 mg, and studies have found that doses of 5,000 mg or higher can trigger symptoms like nausea, diarrhea, headaches and bloating. Vitamin C can also interact with certain medications. 

The good news is that vitamin C-rich foods have a much lower risk of medication interactions than supplements. Still, if you have certain conditions, like HIV, hemochromatosis or kidney disease or are undergoing chemotherapy, Bourgeois and Hakimi recommend keeping an eye on your intake of vitamin C-rich foods to minimize potential side effects. Before taking any vitamin C supplements or boosting your intake through foods, it’s important to talk to a healthcare provider for personalized advice. 

How Much Vitamin C Is Safe Per Day?

The recommended daily allowance (RDA) for vitamin C is based on age and gender. Generally speaking, men should aim for 90 mg daily, while women need approximately 75 mg per day. People who are pregnant or breastfeeding have increased needs and are recommended to consume between 80 mg and 120 mg daily. Because smoking increases oxidative stress, people who smoke should consume an additional 35 mg per day.

To reduce your risk of side effects, like diarrhea or kidney stones, it’s best to stay below the upper limit of 2,000 mg per day—the equivalent of eating 24 oranges. Vitamin C recommendations may also be lower for people taking certain medications or those with certain pre-existing conditions. When in doubt, speak with a healthcare professional. 

The Bottom Line

Vitamin C is an essential nutrient with many health benefits. However, it isn’t safe for everyone, especially when taken in supplement form. If you have a medical condition or are taking certain medications, it’s best to talk with a healthcare professional before adding vitamin C to your wellness regimen. They can review your medical history, order blood work and consider any medications you may be taking to help you make an informed decision.