On average, half of adults are taking at least one prescription medicine per day, and a quarter take at least three drugs, rising to five or more in those of pension age. Surveys also suggest that almost one in two adults take a vitamin, mineral or herbal supplement. The older you are, the more likely it is that you are taking both a prescribed medicine and a food supplement which can lead to problems as many prescribed drugs interact with foods and supplements. Some of these interactions are beneficial, but most have the potential to cause harm.
- How drugs and supplements interact
- How to interpret drug supplement interactions
- Potentially harmful drug supplement interactions
- Warfarin and vitamin K interactions
- St John’s wort and drug interactions
- Cannabidiol CBD oil and drug interactions
- Beneficial drug supplement interactions
- Probiotics and antibiotics
- Vitamin K and antibiotics
- Bromelain and antibiotics
- Echinacea and anti-fungal creams
- Milk thistle and paracetamol
- Vitamin C and paracetamol
- Coenzyme Q10 and statins
- Plant sterols and statins
- Zinc and ACE inhibitors
- Calcium, vitamin D and corticosteroids
How drugs and supplements interact
Drugs and supplements can interact in several ways, to increase or reduce their:
- absorption into the body
- effects inside cells
- activation or inactivation in the liver
- speed of departure from the body via the kidneys or intestines.
The drugs most usually involved in interactions are anticoagulants (eg warfarin, aspirin), sedatives, antidepressants, antiviral drugs, and some medicines prescribed to treat heart problems, high blood pressure and epilepsy.
In some cases, certain supplements may stop a prescribed medicine from working properly, so you lose its benefits. In other cases, the interaction could cause blood levels of the drug to rise, increasing the risks of unwanted side effects.
If you are taking any prescribed medicines, it’s vital to check for potential interactions before taking any vitamin, mineral, herbal or other food supplements.
You can try asking your doctor but most won’t know how to check outside of the information generally supplied with a drug, and will err on the side of caution by advising you not to take the supplement. You will likely do better by asking a pharmacist who generally have more robust interaction checking tools to hand.
You can also check yourself, using the excellent on-line Drug Interactions Checker available from Drug.com. This includes a large database of drug names as well as commonly used vitamins, minerals, herbal and other supplements. It will also alert you to drugs that interact with certain foods or alcohol.
How to interpret drug supplement interactions
When an interaction is flagged as Minor, there is a potential or theoretical chance of an interaction, although no serious effects have been reported. In most cases, you can take the drug and supplement together but should monitor your health – for example, if you have hypertension, monitor your blood pressure more regularly than usual in case it goes low (causing dizziness or faintness) or higher than normal. If changes occur, stop the supplement. Similarly, if you have diabetes, keep a close eye on your glucose readings in case they to higher or lower than normal. If in doubt, check with your doctor.
If an interaction is flagged as Moderate, speak to your doctor before taking the supplement. Your doctor is likely to advise against taking the supplement.
If an interaction is flagged as Major, you should definitely NOT take the supplement.
Do not stop taking any of your prescribed medicine without talking to your doctor, and they advise you can do so.
Potentially harmful drug supplement interactions
All drugs have the potential for food or supplement interactions. The most likely culprits, however, are anticoagulants, sedatives, antidepressants and medicines prescribed to treat heart problems, high blood pressure and epilepsy. Always check before taking a supplement.
Warfarin and vitamin K interactions
Warfarin is a blood thinner that interacts with more drugs and supplements than any other medicine. Warfarin works by blocking the effects of vitamin K, which is needed to produce blood clotting proteins in the liver. As long as you maintain a fairly constant intake of vitamin K your blood clotting tests (INR) will remain stable – it is significant changes in intake (eg suddenly stopping or starting a vitamin K supplement) rather than total intake that causes problems with warfarin control. If you want to start or stop a multivitamin that includes vitamin K, liaise with your warfarin clinic first as you may need more frequent monitoring of your INR.
Supplements known to potentially interact with warfarin or to have additional blood thinning effects (and which may also apply to long-term aspirin) include (but is not limited to):
- Danshen (Salvia species) used to help menstrual irregularity and to relieve bruising
- Dong quai (Angelic sinensis) used to treat menstrual cramps, irregular periods and menopausal symptoms
- Black cohosh
- Fish oil
- Pycnogenol (pine bark extracts)
- Red Clover
- Devil’s Claw
- Ginkgo biloba
Grapefruit juice and Cranberry juice can also cause a modest rise in INR in some people taking warfarin.
St John’s wort and drug interactions
Among the herbal medicines, St John’s Wort has one of the longest lists of potential drug interactions. According to Drugs.com, at least 879 drugs are known to interact with St John’s Wort, of which 248 are major interactions, and 586 are classed as moderate. This is because St John’s Wort stimulates the production of enzymes (cytochrome P450 family and intestinal P-glycoprotein) that are involved in breaking down prescribed drugs in the liver, gut and other organs. As a result, blood levels of some drugs may increase (due to reduced breakdown) causing side effects, while the effectiveness of other drugs may decrease (due to slower production of more active metabolites) when taking St John’s Wort. An important potential interaction is with the oral contraceptive pill, which may increase the risk of unplanned pregnancy.
Cannabidiol CBD oil and drug interactions
Cannabidiol (CBD) extracted from industrial hemp plants also interacts with the cytochrome P450 family of enzymes and blocks their action, especially one known as CYP3A4. This enzyme is itself blocked by some prescribed drugs (eg ketoconazole, itraconazole, ritonavir and clarithromycin) so that levels of cannabidiol increase. Other drugs stimulate production of more of this enzyme (eg rifampicin, carbamazepine and phenytoin) so that levels of CBD are quickly reduced.
The interactions seen with cannabidiol are similar (but stronger) than those seen with grapefruit juice. If the patient information leaflet that comes with your medicine advises against drinking grapefruit juice, then do not take cannabidiol while you are on that medicine. To check other drugs, you can use the Drugs.com Interaction Checker, although this does not specifically include cannabidiol. Instead, you will need to check interactions for the closely related ‘Cannabis’ which is included and, although different to cannabidiol (as it includes another cannabinoid called THC which is psychoactive) it will give you a good steer on whether interactions are known. If in doubt, check with your doctor or pharmacist.
Beneficial drug supplement interactions
It’s not all bad news as some supplements boost the effects of prescription medicines, or replenish the nutrients they deplete.
Probiotics and antibiotics
As well as killing harmful bacteria, broad-spectrum antibiotics also wipe out large swathes of beneficial bacteria in the gut. This can lead to diarrhoea and has been linked with the development of irritable bowel syndrome. Taking a probiotic supplement during and after a course of antibiotics helps to replenish these beneficial bacteria to reduce intestinal side effects.
Vitamin K and antibiotics
Long-term use of antibiotics (eg for acne, or following spleen removed) interferes with the action of vitamin K in the body. A multivitamin supplement that includes vitamin K is therefore a good idea (as well as a probiotic).
Bromelain and antibiotics
Bromelain, a pineapple enzyme, increases blood levels of the penicillin drug, amoxicillin, and improves its ability to penetrate tissues when treating sinusitis, bronchitis and abscess.
Echinacea and anti-fungal creams
Echinacea boosts immunity against infections. One study found that using Echinacea tablets together with econazole anti-fungal cream reduced the rate of recurrence of Candida yeast infections compared with using the cream alone.
Milk thistle and paracetamol
Paracetamol can damage the liver by lowering levels of a liver antioxidant called glutathione. Milk thistle extracts raise glutathione levels, to protect liver cells from toxins, including the effects of paracetamol (and alcohol).
Vitamin C and paracetamol
Vitamin C prolongs the time paracetamol stays in the body. That means you need fewer paracetamol doses per day for pain relief when taking vitamin C, and can spread doses further apart so you need less painkillers overall. Never take more than the recommended dose of paracetamol.
Coenzyme Q10 and statins
Statins lower cholesterol levels by inhibiting the liver enzyme, HMG-CoA reductase. This enzyme is also needed to make coenzyme Q10 for energy production in cells. Statins can halve blood levels of coenzyme Q10 within two weeks, which may contribute to the muscle aches and weakness that often occur in people taking a statin drug. Taking coenzyme Q10 may help to overcome this statin related side effect. Vitamin D levels are also lowered by statin therapy because it is made from a cholesterol-like molecule in the skin on exposure to sunlight. Taking a vitamin D supplement is also a good idea.
Plant sterols and statins
Plant sterols lower cholesterol in a different way to statin drugs. Plant sterol supplements can be added to statin therapy to improve their effectiveness when statins alone do not control your cholesterol readings.
Zinc and ACE inhibitors
ACE inhibitors used to treat high blood pressure and heart failure can lead to a zinc deficiency, especially when used together with a diuretic (water tablets). Lack of zinc can lead to changes in taste sensation and reduced immunity. A multivitamin and mineral supplement that includes zinc is therefore a good idea.
Calcium, vitamin D and corticosteroids
People taking long-term corticosteroids are at increased risk of osteoporosis. Good intakes of calcium and vitamin D are vital to help maintain bone density and are often prescribed together with these medications.
Have you experienced any interactions between supplements and the drugs you are taking?