Beware of recently identified interactions with drugs…

By now, you know that the supplements you pop to stay healthy may turn harmful if you also take certain prescription and/or over-the-counter medications.

What you may not realize: Scientists are still uncovering what the interactions are—and just how dangerous they can be. What you need to know to stay safe…


When researchers at the University of Minnesota recently looked at data from more than 23 million scientific studies, they identified thousands of potential drug–supplement interactions—including some that have only recently been recognized.*  

The danger zone: Some supplements increase drug levels by slowing their breakdown in the body. Some accelerate drug metabolism/breakdown and reduce the desired effects. Other interactions are additive: Drugs and supplements can act on similar pathways in the body and increase the overall effects—and the risk for side effects.


In the meta-analysis mentioned above, researchers discovered that echinacea, a popular herbal remedy for colds and other infections, reduced the activity of exemestane (Aromasin), a drug used for breast cancer. In fact, echinacea interferes with a number of chemotherapy drugs, including cyclophosphamide and fluorouracil. Other drug–supplement interactions…

Iodine. Most Americans get enough iodine from salt, seafood, whole grains and other foods. But some people take supplements because they believe that extra iodine will improve thyroid health. The truth is, the supplement only helps if there’s a true iodine deficiency.

Serious interaction: Levothyroxine (Synthroid and Levoxyl), a synthetic form of thyroid hormone that treats low thyroid (hypothyroidism). High doses of supplemental iodine—300 micrograms (mcg) or more—can interfere with thyroid function. When this happens, a dose of levothyroxine that was previously effective can suddenly stop working.

My advice: Do not take supplemental iodine unless you have been shown (via urine or blood tests) to be deficient, and your doctor OKs it. Supplements often contain 500 mcg to 1,000 mcg of iodine—far more than the recommended daily allowance of 150 mcg.

Helpful: Be cautious when taking any supplement that’s dosed in micrograms. Anything that’s measured in millionths of a gram requires careful minimal dosing.

Fish oil. It has a number of proven benefits—lowering very high triglycerides (500 mg/dL and above)…improving pain from rheumatoid arthritis…slowing the progression of lupus…and even easing mild-to-moderate depression.

Serious interactions: All blood thinners—including not only the popular prescription blood thinner warfarin and newer blood thinners, such as Eliquis and Xarelto, but also over-the-counter drugs with blood-thinning effects, including nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen (Motrin).

Fish oil has a blood-thinning effect because it inhibits the ability of platelets to stick together and form clots. This can be beneficial since blood clots in the arteries are the main cause of heart attacks. But combining fish oil with other blood thinners can cause excess bleeding during surgery or dental procedures…or from wounds or internal injuries (such as ulcers).

My advice: Ask your doctor if you can take fish oil along with your usual blood thinner. The blood-thinning effects of fish oil are dose-dependent—you’re less likely to have problems at typical doses of, say, 2,000 mg or less daily.

Don’t make this mistake: If you’re taking fish oil for high cholesterol, stop. A lot of my patients have been told that it lowers LDL “bad” cholesterol. Not true. At doses of 1,000 mg or more, it can actually raise LDL five to 10 points or more.

GABA (gamma-aminobutyric acid). It’s a neurotransmitter that’s present in the brain and other parts of the body. In supplement form, it has a calming effect and is thought to lower cortisol, the body’s main stress hormone.

Serious interactions: Sedative drugs, including opioids (such as codeine) and antianxiety medications, such as lorazepam (Ativan) or alprazolam (Xanax). Taking these drugs with GABA can cause excessive sedation.

My advice: Never combine sedatives—whether they’re “natural” or pharmaceutical—without checking with your doctor.

St. John’s wort. This herbal supplement has been shown to be as effective as prescription antidepressants in treating mild-to-moderate depression—and with fewer side effects.

However, when combined with SSRI (selective serotonin reuptake inhibitor) antidepressants, such as escitalopram (Lexapro) or paroxetine (Paxil), or other types of antidepressants, the supplement can cause medication-induced serotonin syndrome. This dangerous “overdose” of serotonin, a neurotransmitter that affects mood, can cause swings in blood pressure and heart rate, along with such symptoms as heavy sweating, diarrhea and extreme agitation. But that’s not all.

Serious interactions: An increasing body of evidence shows that St. John’s wort can interact with many other prescription drugs, including warfarin, digoxin and other heart medications, antiseizure drugs, certain cancer drugs and birth control pills. You must let your doctor know if you’re taking St. John’s wort.

L-arginine. L-arginine increases blood levels of nitric oxide, a naturally occurring molecule that dilates blood vessels and can reduce blood pressure by 20 points or more. Some men take it for erectile dysfunction (ED).

Serious interactions: L-arginine can interact with all prescription blood pressure medications, causing blood pressure to drop to dangerously low levels, resulting in dizziness, blurred vision or even a loss of consciousness. The supplement can also cause dangerous drops in blood pressure in men taking ED medication.

My advice: Always check with your doctor before taking a blood pressure drug or ED medication with L-arginine.

*To search for drug–supplement interactions, go to the National Library of Medicine’s website…or the fee-based Natural Medicines Comprehensive Database,