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Negative effects of herbal medicines
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.
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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).
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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.
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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.
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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 art icle in our series about complementar y and alternative therapies.
Cli ck on the links below to r ead the others:
Here’s why we should research alternative therapies
Can we scienti f icall y test herbal medicines?
Yes: Quality research of herbal medicines is possible
No: We can’t have reliable evidence for herbal therapies