Chemotherapy Drug Interactions: What You Need to Know Before Starting Treatment

When you're undergoing chemotherapy, a treatment that uses powerful drugs to kill cancer cells. Also known as cancer chemotherapy, it's not just about the main drug you're given—it's about everything else in your body that might react with it. Many people don’t realize that common medications, supplements, even over-the-counter painkillers can interfere with how chemotherapy works. Some make it less effective. Others turn deadly side effects into emergencies.

Drug interactions, when two or more substances affect each other’s behavior in your body are especially risky with chemotherapy because your system is already under stress. For example, taking blood thinners like warfarin or DOACs with certain chemo drugs can spike your bleeding risk. Or mixing anti-nausea meds with chemo might slow down how fast your liver clears the cancer drugs—leading to toxic buildup. Even something as simple as St. John’s wort, a popular herbal remedy for mood, can drop chemo levels so low that the treatment fails. And then there’s chemotherapy side effects, the unintended reactions from treatment like fatigue, low blood counts, or nerve damage—some of which get worse when combined with other drugs you didn’t think mattered.

You might be taking meds for high blood pressure, diabetes, or arthritis. Maybe you’re on antibiotics after a dental procedure. Or you’ve started a new vitamin because you "feel run down." None of these are harmless in the context of chemo. Your oncologist needs a full list—not just prescriptions, but supplements, herbal teas, and even recreational drugs. A single interaction can delay your next cycle, force a dose reduction, or send you to the ER. That’s why knowing what to avoid is just as important as knowing what to take.

Below, you’ll find real-world examples from patients who’ve dealt with these issues—what went wrong, what worked, and what to watch out for before your next appointment. Whether you’re just starting treatment or you’re in the middle of it, this collection gives you the facts you need to stay safe and in control.