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Risky herb & drug combinations

In Health and Healing by Bec Farah

With at least two out of three Australians using alternative medicine, global criticism regarding the safety of nutritional and herbal medicine is gaining attention.

You may have heard on the radio that vitamin E is a cardio-protective, or seen a television ad highlighting the immune boosting properties of echinacea root, and then headed to your local pharmacy or health food store to purchase these glorious supplements. But would you have first asked yourself a couple questions, such as: “What medications am I currently taking?” “Do I take these on an empty stomach, or with a meal?”

Many people believe that all ‘natural’ herbs and foods are safe with the concurrent use of medication. This is incorrect. Herbs and foods may interact with medication, resulting in serious adverse reactions. Conversely, while certain medications interact adversely with herbs and supplements, there are some herb/drug combinations that can produce therapeutic benefits.

Foods and beverages

Some foods and beverages, even healthy ones, can reduce the therapeutic efficacy of your medication if ingested concurrently.

The presence of food in the digestive tract may delay or decrease the absorption of medications. The absorption of the common antibiotic azithromycin, for example, is decreased when administered with food, reducing its bioavailability by 43%. It is therefore often recommended to take certain medications on an empty stomach or one to two hours after eating.

In contrast, some medications are easier to tolerate when taken with food. Non-steroidal anti-inflammatories (NSAIDs), oestrogen, and cholesterol medications are examples of drugs that should be administered with food to reduce gut irritation and possible nausea.


Dietary fibre can affect medication absorption. Soluble fibres such as pectin have been shown to slow down the absorption of acetaminophen, a common painkiller. Insoluble fibres, including bran, have a similar effect on digoxin, a major heart medication.

Grapefruit juice

Compounds found in grapefruit have been shown to inhibit cytochrome P450, an enzyme in the liver, leading to increased blood levels of medications and toxicity. Adverse reactions such as light-headedness, headaches, and facial flushing are common, and raised blood pressure and heart rates can also result. Grapefruit and its juice are therefore recommended to avoid completely when taking some medications.


Soy milk, tofu, tempeh and soy-containing products raise concerns as consumption has been associated with reduced absorption of thyroid medication.


The amino acid tyramine – found in aged cheeses, red wines, cured meats, and some fermented foods, regulates blood pressure. Monoamine oxides (MAO) inhibitors, commonly prescribed for depression, are known to interact with foods containing tyramine, leading to severe hypertensive emergency. Therefore, the above foods high in tyramine should be avoided altogether with medication administration.


Coffee, black tea, and even chocolate can have a stimulatory impact when taken with certain medications. Patients taking anti-arrhythmic drugs may experience an irregular heartbeat while consuming caffeine. Caffeine can increase anxiety and reduce drug effectiveness of benzodiazepines; reversing the need to sleep. It has been reported that caffeine has raised serum levels of theophylline, a common bronchodilator, by 20-30%, causing nervousness, tremor, or insomnia.


Alcohol interacts with almost every medication and should therefore be avoided with concurrent use. It affects medications through its ability to change the liver’s capacity to filter medication through the body by either increasing or decreasing its effect. Alcohol can enhance the central nervous system’s depressant effects of benzodiazepines and antidepressants, producing excessive drowsiness and incoordination. As a gut irritant, alcohol, in combination with other irritants, including NSAIDs, may increase the risk of gastro-intestinal bleeding.

Nutritional supplements

Vitamins, minerals, essential fatty acids, and amino acids generally have fewer risks in conjunction with medication administration as they are normally found in the body, where they are required for physiological functions.

Exceeding dosages of certain nutritional supplements, however, raises concerns for concurrent use of certain medications.


5-hydroxytryptophan (5HTP) is a naturally occurring amino acid and precursor to serotonin and melatonin. If it’s combined with selective serotonin re-uptake inhibitor (SSRIs) medications (commonly prescribed anti-depressants), or even St John’s wort, can result in serotonin syndrome. This is a potentially life-threatening drug reaction from excess serotonin production and can cause symptoms that range from mild – shivering and diarrhoea – to severe, such as muscle rigidity, fever, and seizures.

Vitamin E

Vitamin E naturally exhibits blood-thinning effects. Daily administration exceeding 1000IU increases the risk of excess bleeding and should therefore be used with caution when combined with blood-thinning medication including warfarin.

Vitamin K

Supplementation and/or diets high in vitamin K such as green leafy vegetables (kale, spinach, broccoli and brussels sprouts) can counteract the effectiveness of blood thinning medication, increasing the risk of a stroke. It is recommended to check nutritional supplement lists to avoid the mistake of doubling up. As warfarin interacts with vitamin K in the body, it is important to keep vitamin K intake, supplemental or dietary, consistent each day.


Similarly, supplementation and/or diets high in iodine, including seaweed and seafood, can lower thyroid drugs’ efficacy. Caution is warranted when taken with thyroid medication.

Herbal supplements

As herbs have similar actions to medications and can potentially cause toxicity and adverse reactions, it is recommended for individuals to be under the care of a healthcare practitioner with clinical experience in herbal medicine.

Laxatives including senna, cascara, and aloe increase gut motility, thereby accelerating the elimination of drugs. Sedatives such as kava, hops, and valerian potentiate further sedative effects.

In some cases, however; herbs and medications have shown to have a synergistic effect or prevent adverse reactions. Ginger, for example, can be used to avoid drug-induced nausea, while St Mary’s thistle can be taken to protect the liver from toxicity associated with drugs. Garlic also proves to prevent toxic metabolites produced from paracetamol.

St John’s wort

Commonly known for the treatment of anxiety, insomnia, and as a ‘natural anti-depressant’, St John’s wort stimulates the production of liver enzymes, resulting in lower concentrations of medication in the body, therefore should be avoided with any type of medication. Interactions with SSRIs, such as Prozac , can potentially lead to serotonin syndrome. Administering St John’s wort with warfarin can increase your susceptibility to blood clotting, strokes, and heart attack. Activation of liver enzymes by St John’s wort reduces cholesterol medication levels, elevating cholesterol in the blood. It also reduces the levels of oral contraceptives – which can potentially lead to accidental pregnancies.


Renowned for improving cognitive function and circulation, ginkgo can thin the blood, interacting adversely with blood thinning medications including warfarin and aspirin. High doses of ginkgo have also been reported to reduce the effectiveness of anticonvulsant medications to control seizures. Caution is thereby warranted when taken with medications that thin the blood and are central nervous system depressants.


Similar to ginkgo, garlic contains properties that can also thin the blood, so caution is advised when taking with blood thinning medications.


Don’t get too excited! This is not referring to the sweet treat, but rather the root of a liquorice plant that can be used for various ailments from cold sores to stomach ulcers and soothing a sore throat. Liquorice has the potential to increase water retention by affecting levels of sodium and potassium in the body. Use caution when taking it with diuretic (urine-producing) medications. Liquorice can also interact with digoxin used for congestive heart failure and regulating heart rhythms, increasing the risk of digoxin toxicity.


Universally used for treating the common cold and acute infections, echinacea can interact with fexofenadine; used for allergy symptoms, itraconazle; an anti-fungal medication, as well as lovastatin; a commonly prescribed cholesterol medication. Increasing the effects of these medications can make you more susceptible to liver toxicity, and echinacea reduces their effectiveness in the body.

Korean ginseng

Korean ginseng, a Chinese medicinal herb commonly used for boosting physical and mental performance, as well as erectile dysfunction, can interact with pain-relieving medication, various diuretics, and blood pressure medication, reducing the effectiveness of pain perception as well blood pressure levels. Additionally, Korean ginseng has also been found to reduce the effects of warfarin and metformin, a commonly prescribed blood sugar medication; so take care with concurrent use.


Kava has had a long history of use for stress, insomnia, and anxiety. It has been found to have additive effects with central nervous system depressants including benzodiazepines and antipsychotics, making you prone to feeling lethargic and disoriented. Avoidance of kava with central nervous system depressants is therefore advised.


Checklist when considering herb-drug interactions

  • Read ingredients, directions, warning and precautions printed on supplements and medications. If you do not understand something or need more information, ask your healthcare practitioner.
  • Avoid taking vitamin supplements with medications as they can interact with certain drugs.
  • Take supplements and medications with a full glass of water.
  • Never take vitamin supplements and medications with alcohol.
  • Ensure to inform your health care practitioner of any supplement and medication you are currently taking.
  • Check with the pharmacist on how food can affect specific medications taken with food.
About the author

Bec Farah

Bec Farah is a naturopath, nutritionist, herbalist, fitness instructor, writer, speaker. She’s the founder of ‘Bec to Nature Naturopathy’. Her mission is to educate people on how to get back to the basics, by getting back to nature. She has a special interest in skin health and her passion is to adopt food as medicine. http://www.bectonature.com.au


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