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Herbal medicines – toxic side effects and drug interactions

TESTING ALTERNATIVE THERAPIES - La Trobe University’s decision to accept funding from Swisse for a new centre to research alternative medicines has sparked controversy. This series looks at how the evidence…

Most people don’t think of herbal medicines as drugs so they don’t tell their doctor about taking them. Ryan Somma/Flickr

TESTING ALTERNATIVE THERAPIES - La Trobe University’s decision to accept funding from Swisse for a new centre to research alternative medicines has sparked controversy. This series looks at how the evidence behind alternative medicines can be assessed, and the ethics of such links between industry and research institutions.


The simple fact that herbal medicines are drugs is underappreciated or not understood at all by most people. They include good drugs, bad drugs and completely useless drugs, but they are drugs nonetheless, and therein lies a lot of grief.

Herbal medicines are often promoted as “natural” (and therefore gentle). People tend to think of a relaxing cup of chamomile tea made from ingredients hand-picked by benevolent beaming grannies in an Arcadian setting. But while the implication is that “natural” is good, the majority of the most toxic compounds we know of are natural.

Natural toxins

Botulinum toxin is 100% natural and the most deadly substance known on this planet. Yet suitably diluted Botulinum toxin is used to relieve intractable muscle spasms (as well as make the skin less wrinkly by paralysing muscles with Botox).

While the vast majority of herbal medicines are not as dramatically lethal as Botulinum toxin, virtually all will have some adverse effects. Indeed, any substance that alters your body’s physiology will have side effects.

After all, the main reason that there so many chemicals in plants that we can use as medicines is that they are used as defences. So they interfere with some aspect of physiology to stop animals eating them (morphine, caffeine, Tetra-Hydro Cannibidiol in marijuana and those lovely aromatic oils in Eucalyptus oil are some well-known examples). Or they provide defence against infection (salicylic acid in willow bark, for instance).

So you would expect some side effects.

Extract of foxglove is effective for treating heart failure, but get the concentration wrong and it is lethal; willow bark is effective for reducing pains and fever but causes ulceration of the mouth and throat if used for a while; and Senna pods are used to relieve constipation, but can cause heart problems and gastrointestinal damage.

Drug interactions

What’s more concerning is that since people don’t think of herbal medicines as drugs, they also don’t consider they will interact with their other medicines. But they do.

St John’s Wort enhances the effects of any prescribed antidepressants, leading to potentially lethal overdose. John Tann/Flickr, CC BY

The poster child for drug-herb interactions is St. John’s Wort – because it has a twofold effect. The compounds in it that are responsible for its antidepressant effect will enhance the effects of any prescribed antidepressants, leading to potentially lethal overdose.

Not only does St. John’s Wort interact with antidepressants, the chemicals in it also stimulate the liver to break down certain classes of drugs more rapidly. People taking the herb have been endangered because their anti-rejection drugs or anti-HIV drugs have been broken down below effective therapeutic levels.

Despite the side effects of St. John’s Wort being well known in the pharmaceutical community, information about them from points of sale are generally very poor and most consumers will be unaware of them (see also here).

And it’s not just St. John’s Wort. A whole range of herbal medicines interact with conventional medicines (dandelion and diuretics is another example). This puts people in harms way when they are prescribed conventional drugs.

Because most people don’t think herbal medicines are drugs, they tend not to tell their medical practitioner about their herbal use. And medical practitioners tend not to ask about specifically about it as they expect their patients will tell them!

Missing ingredient

But if herbal medicines have such a range of adverse effects, why don’t we see more evidence of them? There are three reasons for this.

First, although figures for complementary medicine show roughly half of Australians taking complementary medicines, these are mostly vitamins (which have their own problems). Far fewer people actually take herbal medicines.

In a study of antidepressant use in New South Wales, for instance, very few people were taking St. John’s Wort, and even fewer were taking it together with conventional antidepressants.

Second, adverse reactions to herbal medicines are significantly under reported. While there is a “blue card” system for reporting adverse events to conventional medicines, herbal medicines are often bought from health-food stores, or prescribed by herbalists who don’t participate in the system.

So there’s no way for most herbal medicine buyers to report adverse events.

Extract of foxglove is effective for treating heart failure, but get the concentration wrong and it is lethal. e_chaya/Flickr, CC BY

Finally, there’s some evidence that many herbal medicines have very low amounts of the active ingredients, or do not have the active ingredient at all. So many herbal medicines are no more than expensive placebos.

Learn to distrust

Most herbal medicines are classified as “Listed” by the Therapeutic Goods Administration. This means that unlike registered medicines such as paracetamol and statins, the evidence required for approval is much less stringent.

In fact, it’s basically an honour system where the herbal medicines sponsor says there’s no evidence of harm, and we hold documentation that shows this. Mostly, the evidence is historical, claiming that people have been using it for generations without evidence of harm.

But a long history of use is not necessarily evidence of no harm. Willow bark, for instance, has as long a history of causing ulcers as it has of relieving pain, but its serious side effects have not passed into lore.

Until we used modern medical investigation, we were unaware of the harms caused by the herb borage, and the severe kidney toxicity and cancer caused by the Aristolochic acid found in herbs used in traditional medicine.

Herbal medicines are widely trusted but that trust is mostly due to our imagination coupling them with a bucolic vision of nature which never existed. It’s time to end this misplaced trust and start seeking evidence.

We still only have a very poor idea of the potential harms posed by the panoply of herbal medicines on sale.

This is the second article in our series about complementary and alternative therapies. Click on the links below to read the others:

Can we scientifically test herbal medicines?