Author: Dr Sarah Brewer MSc (Nutr Med), MA (Cantab), MB, BChir, RNutr, MBANT, CNHC, FRSM Category: Health, Men's Health, Supplements & Vitamins, Women's Health

Some herbal medicine may interact with prescription medicines, while others may work well together, Dr Sarah Brewer explains

A recent story that researchers at the University of Hertfordshire warned that mixing some herbal remedies with prescription medication can increase the risk of bleeding, raise blood sugar levels or could stop medications from working effectively.  While many interactions are theoretical, based on limited evidence, or only occur at high doses, it is really important to check for interactions before you mix tablets and supplements.

It’s not just mixing medications, people are often unaware that some foods and drinks taken alongside medication could dramatically increase how the body absorbs them or render them effectively useless.  But it’s not all bad news.  At the other end of the scale, there are many drug and supplement combinations that can actually work well together – sometimes dramatically boosting the effectiveness of one or both ingredients.

Chris Etheridge, Chair of the BHMA says, “Whilst it is well known that in a few well documented cases, herbal medicines can affect the way prescribed and over-the-counter medicines work – such as St John’s Wort – there are very few other herbal medicines that affect medicines in this way”.
He goes on to say, “Anyone that is considering taking a herbal medicine should always buy one that displays the THR logo on the pack.  It is important that you read the THR in-pack leaflet before taking the herbal medicine in order to ensure that it is safe and appropriate to do so.  If you are taking another medicine, either prescribed by your doctor or over-the-counter, it is also sensible to double check its leaflet to ensure that it is safe to take the medicines and herbal medicine together.  The information contained in these leaflets is accurate and reliable and has been approved independently by the UK medicines licensing authority, the Medicines and Healthcare Products Regulatory Agency.”  For more information on how to check if your herbal medicine can be taken with your prescribed medicine go to

Herbal medicines and food supplements can interact with drugs in several ways, to increase or reduce their:

  • absorption into the body
  • effects inside cells
  • break down in the liver
  • speed of departure from the body via the liver, kidneys or intestines

The effects of an interaction can range from the drug simply not working, to it working too well. 

Another important point is that supplement quality varies widely.  I recommend only selecting products made to an international pharmaceutical standard known as Good Manufacturing Practice (GMP).  This tests products for purity and consistency of dose at every stage of manufacture.  As this standard takes additional time and money, GMP is usually mentioned as a quality point in product write ups and on the pack, such as with Healthspan* products.  You can also try searching on the internet for the name of your chosen brand plus GMP.  

This is especially important for herbal medicines as it ensures the right herbs are included – there are several cases of Chinese herbal products containing misidentified raw ingredients that have resulted in serious liver side effects, for example. 

Potentially Harmful Interactions

All drugs have the potential for food, herb or supplement interactions.  The most likely culprits, however, are anticoagulants (blood thinners such as warfarin or aspirin), sedatives, antidepressants and medicines prescribed to treat heart problems, high blood pressure and epilepsy.

Warfarin and Green Vegetables

Warfarin blocks the effects of vitamin K, which is needed to produce blood clotting proteins in the liver. Foods containing vitamin K include cauliflower (richest source), broccoli, spinach, Brussels sprouts, asparagus, lettuce and avocado.  This is why you need to maintain a fairly constant intake of these foods to ensure your blood clotting control remains stable – it is significant changes in intake, rather than total intake, that causes problems with warfarin control.

Paracetamol/Acetaminophen and Warfarin

Less well known is that paracetamol (acetaminophen) also increases the blood thinning effect of warfarin.  Those with the highest intake of paracetamol (9g per week, or more) while on warfarin have a 10-fold increased risk of over-anticoagulation, which could lead to haemorrhage such as a potentially fatal stroke.  

Be careful with St John’s Wort

A traditional herbal medicinal product used to relieve the symptoms of slightly low mood and mild anxiety, St John’s Wort has one of the longest lists of potential drug interactions.  This is because it affects the liver’s system of enzymes that break down many drugs in the body.  As a result, taking St John’s Wort may cause blood levels of some drugs (including but not limited to, oral contraceptive pills and HRT, antidepressants, blood thinning medicines, statins and drugs used to treat high blood pressure, migraine, HIV and cancer) to increase (causing side effects) or decrease (reducing their effectiveness). 

An important potential interaction is with the oral contraceptive pill, which may increase the risk of unplanned pregnancy.  This is because St John’s Wort increases the production of liver enzymes that breakdown medicines and has been found to lower blood levels of contraceptive hormones by around 15 per cent – enough to cause breakthrough bleeding or ovulation and unplanned pregnancy. 

Be mindful of the below:

CBD (cannabidiol) oil
Good for: relaxation, anxiety, stress, inflammation and pain.
Don’t take if: you are on any prescribed medicines without first checking for interactions – CBD can inhibit liver enzymes involved in metabolising some medicines and may result in increased drug levels that could cause side effects.

Good for: temporary sleep disturbance.
Don’t take if: you are taking sleeping tablets, buprenorphine or other medications that make you feel drowsy.  Avoid excess alcohol.

Black Cohosh
Good for: the relief of symptoms of the menopause, such as hot flushes, night sweats, and temporary changes in mood (such as nervous irritability and restlessness).
Don’t take if: you have a history of liver disease or if you have, or have had, an oestrogen dependent tumour such as breast or ovarian cancer.  Avoid alcohol and other drugs known to affect the liver.

Devil’s Claw
Good for: a traditional herbal medicinal product with good evidence for its painkilling and anti-inflammatory properties which help to reduce back and joint pain – and reliance on conventional painkillers.
Don’t take if: you have a history of stomach or duodenal ulcers or are on warfarin.  Devil’s Claw can usually be taken alongside conventional painkillers but check with your doctor first.

While you do have to be careful with drug and supplement interactions, it’s not all bad news as some supplements actually boost the effects of prescription medicines or replenish the nutrients they deplete such as painkillers and vitamin C which actually boosts the effects of painkillers such as paracetamol, aspirin, and opiate drugs such as morphine.

For further free advice Dr Brewer also recommends checking the  interactions checker at which includes most commonly used herbal medicines, food supplements and prescribed medicines.
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