Capecitabine Drug Interactions You Need to Know About

When you're taking Capecitabine (brand name Xeloda) for cancer treatment, knowing what other medications, supplements, and foods can interact with it isn't just important — it can be life-saving. Capecitabine has a boxed warning about one particular drug interaction, and several others that your oncologist needs to know about.

This guide covers the major and moderate drug interactions, supplements and over-the-counter products to watch, food interactions, and what you should tell your doctor before starting treatment.

How Drug Interactions Work with Capecitabine

Capecitabine is a prodrug — it's inactive when you swallow it and gets converted into the active chemotherapy agent 5-Fluorouracil (5-FU) through a three-step process in your body. Drug interactions can happen in several ways:

• Enzyme interference: Some drugs affect the enzymes that activate or break down Capecitabine, leading to dangerously high or ineffectively low levels of 5-FU.
• CYP2C9 inhibition: Capecitabine inhibits the CYP2C9 enzyme, which means it can increase levels of other drugs that are processed by this enzyme.
• Additive toxicity: Some medications increase the same side effects Capecitabine causes (like myelosuppression or GI toxicity), making the combination more dangerous.
• DPD pathway interference: Certain drugs block dihydropyrimidine dehydrogenase (DPD), the enzyme that breaks down 5-FU, which can lead to fatal toxicity.

Major Drug Interactions (Potentially Life-Threatening)

Warfarin (Coumadin) and Other Blood Thinners

⚠️ BOXED WARNING: This is the most dangerous known interaction with Capecitabine. Taking Capecitabine with Warfarin or other coumarin-derivative anticoagulants can cause clinically significant increases in prothrombin time (PT) and INR. Deaths have been reported.

What happens: Capecitabine inhibits CYP2C9, the enzyme that metabolizes Warfarin. This causes Warfarin to build up in your system, dramatically increasing your risk of bleeding.

What to do:

• If you take Warfarin, your doctor must monitor your INR frequently — potentially weekly or more often during Capecitabine treatment.
• Your Warfarin dose will likely need to be reduced.
• Watch for signs of bleeding: unusual bruising, blood in urine or stool, nosebleeds that won't stop, or bleeding gums.
• This interaction can occur days to months after starting Capecitabine and even after stopping it.

Phenytoin (Dilantin)

Capecitabine inhibits CYP2C9, which is the main enzyme that processes Phenytoin. This interaction can cause Phenytoin levels to rise to toxic levels, leading to:

• Dizziness and difficulty walking
• Slurred speech
• Nystagmus (involuntary eye movements)
• Seizures (paradoxically)

If you take Phenytoin for seizures, your doctor will need to monitor your Phenytoin levels closely and adjust the dose during Capecitabine treatment.

Leucovorin (Folinic Acid)

Leucovorin enhances the toxic effects of 5-FU, which is the active metabolite of Capecitabine. When used together, there is increased risk of severe gastrointestinal toxicity, including diarrhea and mucositis. Deaths have been reported from this combination. Your oncologist is aware of this interaction and will only combine these drugs with careful dose management.

Sorivudine and Brivudine (Antiviral Agents)

These antiviral drugs inhibit DPD, the enzyme that breaks down 5-FU. Taking them with Capecitabine can lead to fatal 5-FU toxicity. A minimum 4-week washout period is required after stopping Sorivudine or Brivudine before starting Capecitabine. While these antivirals are more common outside the U.S., this interaction is critical to know about.

Moderate Drug Interactions

CYP2C9 Substrates

Because Capecitabine inhibits CYP2C9, it can increase the blood levels of many medications processed by this enzyme. Beyond Warfarin and Phenytoin, these include:

• Celecoxib (Celebrex) — a common anti-inflammatory
• Losartan (Cozaar) — a blood pressure medication
• Glipizide and Glimepiride — diabetes medications (risk of low blood sugar)
• Fluvastatin (Lescol) — a cholesterol medication

Your doctor should review all medications you take that are CYP2C9 substrates and adjust doses or monitor more closely.

Other Antineoplastic (Chemotherapy) Agents

When Capecitabine is combined with other chemotherapy drugs, there is additive risk of myelosuppression — meaning your blood counts (white cells, red cells, and platelets) can drop more severely. Common combinations include:

• Docetaxel (Taxotere) — used with Capecitabine for breast cancer
• Gemcitabine (Gemzar) — used with Capecitabine for pancreatic cancer
• Oxaliplatin (Eloxatin) — used in colorectal cancer regimens

These combinations are intentional and managed by your oncologist with frequent blood work.

Allopurinol (Zyloprim)

Allopurinol, used for gout, may decrease the effectiveness of Capecitabine. If you take Allopurinol, discuss this with your oncologist before starting treatment.

Supplements and Over-the-Counter Products to Watch

Some common supplements and OTC medications can interact with Capecitabine:

Folic Acid Supplements

Folic acid (and Leucovorin, which is a form of folate) can enhance the toxicity of 5-FU/Capecitabine. While routine multivitamins contain small amounts of folic acid, talk to your oncologist before taking any additional folate supplements.

Antacids and Acid Reducers

While not a major interaction, some acid-reducing medications may theoretically affect absorption of Capecitabine. Take any antacids at a different time than your Capecitabine dose.

Blood-Thinning Supplements

Supplements with anticoagulant properties can add to the bleeding risk already increased by Capecitabine. Be cautious with:

• Fish oil / Omega-3 supplements
• Vitamin E (high doses)
• Garlic supplements
• Ginkgo Biloba
• Turmeric/Curcumin

NSAIDs (Non-Steroidal Anti-Inflammatory Drugs)

Over-the-counter pain relievers like Ibuprofen (Advil, Motrin) and Naproxen (Aleve) can increase bleeding risk and may interact with CYP2C9 inhibition by Capecitabine. Acetaminophen (Tylenol) is generally safer for pain relief during treatment — but always check with your doctor first.

Food and Drink Interactions

Food (Take With Meals)

Capecitabine should be taken within 30 minutes after meals. While food reduces the rate and extent of absorption, the prescribing information recommends taking it with food to reduce gastrointestinal side effects like nausea and diarrhea. So this is one case where the food interaction is actually beneficial.

Grapefruit and Grapefruit Juice

Grapefruit can interfere with drug metabolism through enzyme interactions. While the interaction with Capecitabine specifically is not fully characterized, it's generally advised to avoid grapefruit and grapefruit juice during treatment.

Alcohol

There is no specific listed interaction between alcohol and Capecitabine, but alcohol can irritate the stomach and liver, worsen nausea, and contribute to dehydration — all of which can amplify Capecitabine side effects. Most oncologists recommend limiting or avoiding alcohol during chemotherapy.

What to Tell Your Doctor Before Starting Capecitabine

Before your first dose of Capecitabine, make sure your oncologist knows about:

1. Every medication you take — prescription, over-the-counter, and herbal. Don't leave anything out, even if it seems unrelated.
2. All supplements and vitamins — especially folic acid, fish oil, vitamin E, and herbal supplements.
3. If you take blood thinners — Warfarin, Heparin, Eliquis (Apixaban), Xarelto (Rivarelbaban), or any anticoagulant.
4. Seizure medications — especially Phenytoin (Dilantin) or Fosphenytoin.
5. Gout medications — particularly Allopurinol.
6. Recent antiviral use — especially Brivudine or Sorivudine in the past 4 weeks.
7. Kidney and liver function — your doctor will run blood tests, but share any history of kidney or liver problems.
8. Heart history — Capecitabine can cause cardiotoxicity, and certain medications may increase that risk.

Keep an updated medication list and bring it to every oncology appointment. When you start any new medication during Capecitabine treatment, check with your oncologist or pharmacist first.

Final Thoughts

Capecitabine is a powerful and effective cancer treatment, but its drug interactions — especially with Warfarin — require vigilance. The good news is that most interactions are manageable when your healthcare team knows about them upfront.

Don't be afraid to ask questions. Your oncologist and pharmacist are your best resources for understanding how Capecitabine fits with everything else you take. Being proactive about sharing your full medication list can literally be life-saving.

For more information about Capecitabine, read our guides on what Capecitabine is and how it's used, or learn about how to save money on your prescription. If you need to find a pharmacy with Capecitabine in stock, try Medfinder.