Herbs, Supplements and Prescription Drugs


Herbs, Supplements and Prescription Drugs

Two out of three Australians use complementary and alternative medicine services annually. As a result, many Australians also take natural health products (NHPs) alongside prescription medications, making themselves vulnerable to potential adverse reactions and interactions. Patients are best served when they disclose the NHPs and medications they’re taking to their health care practitioners.

Nutritional supplements

Vitamins; minerals; amino acids; and essential fatty acids, such as evening primrose oil, linseed oil and fish oil generally pose fewer risks when combined with medications. They are normally found in the human body, where they facilitate regular biochemical and physiological functionality.

Concerns have been raised about fish oil combined with doses of more than 400 IU vitamin E per day, since both thin the blood and each must be used with caution when combined with medications such as warfarin.

Other genuine concerns involve the combined use of 5-hydroxytryptophan (5-HTP) with antidepressant medications. While 5-HTP (a natural product not sold in Australia, yet widely available in New Zealand) is very safe, if combined with selective serotonin reuptake inhibitor medications (SSRIs) or even St John’s wort, they could cause serotonin syndrome. This is a potentially life-threatening drug reaction that occurs when the body has too much serotonin.

Herbal supplements

Deciding whether to take a herbal product with a medication is more complicated. If individuals are not under the care of a knowledgeable health care practitioner, then they should refrain from combining herbal products with medication.

If, on the other hand, individuals are properly supervised, then it is entirely possible to combine herbal products and medications based on a thorough evaluation by the health care practitioner.

Many clinical scenarios benefit from using herbal products and medications together. For example, diabetic patients might benefit from taking Korean red ginseng or Panax ginseng because this herb can lower blood sugar levels and might allow patients to rely on less medication.

The herb valerian can be combined with benzodiazepines and might enable patients to use less medication. These are a few examples of positive interactions that can result when herbal products are judiciously combined with medication.

However, interactions between herbal products and medication can sometimes result in adverse clinical outcomes.

St John’s wort

Used to treat depression, anxiety and insomnia, the majority of St John’s wort products inhibit a common enzyme in the liver that is largely responsible for metabolising most medications, so it should never be taken with any type of medication.

Combining St John’s wort with warfarin, a common blood thinner, can result in decreased clotting times, making individuals more vulnerable to clotting and cardiovascular events, such as strokes and heart attacks. St. John’s wort can also reduce the concentrations of statin medications in the blood, resulting in cholesterol levels that remain higher than expected.

This herb can also interact with SSRIs, such as Prozac, and potentially lead to a life-threatening medical emergency, known as serotonin syndrome. This condition is characterised by mental status changes, diarrhoea, increased body temperature and blood pressure fluctuations.

St John’s wort extract also reduces the levels of oral contraceptives, which could lead to an unwanted pregnancy.


Used to treat conditions ranging from the common cold to cancer, ginseng, especially the Korean red or Panax type, can result in increased blood pressure. Individuals on blood pressure medication should be cautious since their concomitant use might result in poor blood pressure control.

Korean ginseng or Panax ginseng can also reduce the therapeutic effects of warfarin and might lower blood sugar levels among patients taking antidiabetic medication, such as metformin.


Known for its antioxidant, antifungal and antiageing properties, garlic also has blood-thinning (anticoagulant) properties, so caution is warranted when taken alongside medications that thin the blood.


Used to treat heart disease and circulatory health, when taken with heart medications, hawthorn will increase the therapeutic effects since it improves the heart’s contractile function and even opens up (vasodilates) blood vessels. However, hawthorn can also increase the risk of bleeding among patients on blood-thinning medications, so caution is warranted.

Saw palmetto

Used as a treatment for alopecia and various bladder and prostate conditions, saw palmetto can increase bleeding when taken alongside blood-thinning medication. This herb also > increases the effects of aspirin and other nonsteroidal anti-inflammatory medications, because it inhibits a specific enzyme called cyclooxygenase in the body.


Used by many to treat the common cold, among other conditions, echinacea can increase the effects of statin medications, making individuals more vulnerable to liver toxicity. Compounds in echinacea might also influence liver metabolism, making medications less effective.


A remedy for stomach ulcers, cold sores and canker sores, licorice can increase water retention by influencing sodium and potassium levels in the body, so caution is warranted when taking it alongside diuretic medications. It can also raise blood sugar levels, making diabetic medications less effective. Licorice also thins the blood, which increases the risk of bleeding if taken with blood-thinning medications.

Black cohosh

Used to treat menopausal symptoms, black cohosh might inhibit the liver’s ability to metabolise medications, rendering medications less effective.


This herb used to treat insomnia, anxiety and depression can increase the sedating properties of benzodiazepine medications, such as lorazepam, and might interfere with SSRI medications.

It’s all in the timing

Prior to surgery

To be safe, all NHPs should be discontinued at least seven to 10 days prior to any surgical procedure. Reasons for this include blood clotting problems and/or interactions with anaesthesia or other administered medications.

During pregnancy

With respect to pregnancy, it is best if herbal products are avoided altogether. Only a few herbs have some data showing safety during pregnancy. Ginger can be taken for nausea and vomiting during pregnancy, while red raspberry can be taken near term to facilitate normal (vaginal) delivery.

The data is simply not robust enough to clearly state that taking most herbal products during pregnancy is safe. Plenty of data exists showing benefits from specific nutrients during pregnancy, but these should be discussed with a qualified health care practitioner.


Breastfeeding doesn’t hold the same concerns as pregnancy even though herbal products will pass into the breast milk after being absorbed from the mother’s gastrointestinal tract. Consult an experienced lactation specialist or paediatrician with a practice focused on breastfeeding.

No worries

There’s no need to worry about these harmless reactions to nutrients:

  • skin flushing from taking niacin
  • increased yellow colour of the urine caused by ingesting vitamin B2 (riboflavin)

Foods and beverages that may interact

Foods and beverages, when combined with medications, can also result in adverse interactions.

Grapefruit juice should be avoided since it can inhibit the CYP3A4 enzyme in the liver, leading to increased blood levels of drugs and toxicity reactions.

Cranberry juice may possess blood-thinning properties and increase the likelihood of bleeding among patients on warfarin.

Orange juice should be avoided if taking atenolol, ciprofloxacin or fexofenadine.

Tyramine is an amino acid that regulates blood pressure, which is found in aged cheeses, cured meats, sauerkraut, soy sauce and some wines. It must be significantly limited or avoided altogether when taking medications known as monoamine oxidase inhibitors, commonly prescribed for depression. An interaction between the two can lead to a severe hypertensive emergency.

Soy also raises concerns.Soy supplements, soy milk or soy-containing foods can decrease the absorption of thyroid medication.


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