When Plants Meet Pills

Natural doesn't always mean harmless. Plants are powerful precisely because they contain active compounds that change how the body works, and that is also why they can change how our medicines work. Most herbs and foods are perfectly safe alongside most medicines. But a few combinations can make a medicine weaker, stronger or more likely to cause side effects. Knowing the important ones lets you enjoy the best of both worlds safely.

A discovery made by accident

In 1989, Canadian researchers were testing whether alcohol affected a blood pressure medicine called felodipine. To hide the taste of the alcohol, they mixed it with grapefruit juice. To their surprise, the levels of the medicine in the volunteers' blood were several times higher than expected. The cause turned out to be the grapefruit, not the alcohol.¹

We now know that grapefruit contains compounds called furanocoumarins, which block an enzyme in the gut wall called CYP3A4. This enzyme normally breaks down a large share of medicines before they reach the blood. When it's blocked, far more of the medicine gets through. By 2012, more than 85 medicines were known to interact with grapefruit, over 40 of them seriously.² A single glass of juice can have an effect that lasts for a day or more.

How plants and medicines interact

Interactions happen in a few main ways:

• Changing how medicines are broken down. The liver and gut use a family of enzymes, called cytochrome P450 enzymes, to process most medicines. Some plants slow these enzymes down, raising medicine levels, as grapefruit does. Others speed them up, so the medicine is cleared too quickly and stops working properly.
• Adding to the same effect. If a herb and a medicine do the same thing, such as thinning the blood, lowering blood sugar or causing drowsiness, together they may do too much.
• Blocking absorption. Some foods and minerals bind to medicines in the gut so that less is absorbed.

Important interactions to know about

St John's wort

St John's wort is a well-studied herb for mild to moderate low mood. But it is also one of the most important herbs for interactions, because it speeds up the enzymes that break down many medicines.³ It can make these less effective: (See Where Body Care Meets Mind.)

• the contraceptive pill and other hormonal contraceptives, risking unplanned pregnancy
• the blood thinner warfarin
• medicines that prevent organ transplant rejection, such as ciclosporin
• some medicines for HIV, heart conditions, epilepsy and cancer

Taken with antidepressants such as SSRIs, or with triptans for migraine, it can also cause serotonin syndrome, a potentially serious reaction. In the UK, St John's wort products carry warnings for this reason.

Blood thinners

People taking blood thinners, such as warfarin, apixaban, rivaroxaban, clopidogrel or daily aspirin, need to take particular care. Concentrated supplements of ginkgo, garlic, turmeric, ginger, fish oil and high-dose vitamin E may add to their blood-thinning effect, and some have been linked with bleeding.⁴ Using these as foods, in normal cooking amounts, is generally fine. (See Food as Medicine.)

If you take warfarin, foods rich in vitamin K, such as kale, spinach, broccoli and other leafy greens, affect how it works. You don't need to avoid them, but keep the amount you eat steady from week to week, and tell your clinic about any big changes to your diet.

Thyroid medicine

Levothyroxine is absorbed poorly if taken with coffee, calcium, iron, soya, high-fibre breakfasts or antacids. Take it with water on an empty stomach, ideally 30 to 60 minutes before breakfast, and keep iron and calcium supplements at least four hours apart.

Blood sugar medicines

Herbs that lower blood sugar, such as berberine, cinnamon supplements, fenugreek, gymnema and bitter melon, may add to the effect of diabetes medicines and cause blood sugar to fall too low. Berberine also affects the enzymes that break down several other medicines. (See Balancing the Sugar Seesaw.)

Blood pressure and heart medicines

Liquorice root, taken regularly in large amounts, can raise blood pressure and lower potassium, working against blood pressure medicines and adding to the potassium loss caused by some diuretics. Deglycyrrhizinated liquorice (DGL) doesn't carry this risk. Grapefruit affects some statins and calcium channel blockers. Red yeast rice naturally contains a compound identical to the statin lovastatin, so it should never be taken alongside a statin.

Calming and sleep medicines

Herbs that calm or sedate, such as valerian, kava, passionflower, hops and high doses of ashwagandha, may add to the effects of sleeping tablets, anti-anxiety medicines, strong painkillers and alcohol. Ashwagandha may also affect thyroid hormone levels. (See The Hormone Web.)

Antibiotics and minerals

Calcium, iron, magnesium and zinc, whether from supplements, antacids or dairy foods, can bind to some antibiotics, including tetracyclines such as doxycycline and quinolones such as ciprofloxacin, so they aren't absorbed. Leave a gap of a few hours, and check the leaflet.

Plants are powerful precisely because they change how the body works.

Before an operation

Some herbs and supplements can affect bleeding, blood pressure or how anaesthetics work. Doctors usually advise stopping herbal supplements around two weeks before planned surgery.⁵ Always tell your surgical team about everything you take.

Tell your doctor, and your herbalist

Studies have found that most people who take herbs and supplements don't tell their doctor.⁶ Some worry about being judged; others simply aren't asked. But your doctor and pharmacist can only keep you safe if they know the full picture. In the same way, a good natural health practitioner will always ask about every medicine you take, and check each herb for interactions before recommending it.

At Rootwise, we believe natural and conventional medicine work best side by side. Every recommendation we make is checked against the medicines you take, so you can benefit from the wisdom of plants with confidence. (See also The Architecture of Balance.)