Revealed: The mind-boggling drug interactions you MUST know - and it's not just grapefruit juice and statins you need to beware of...

If you are taking more than one medication, you've probably been warned about dangerous drug interactions you need to avoid.  

But you may not be aware that it's not just mixing drugs we need to be careful of... And it's not just grapefruit juice and statins. 

Grapefruit juice and statins

Statins are a medication commonly prescribed to help prevent heart attacks and strokes. 

They help to lower LDL cholesterol, the 'bad' kind.

Elevated levels in the blood can harden and narrow arteries. 

People advised to take statins are also often encouraged to make healthy lifestyle changes, such as adding more fruits and vegetables to their diet. 

But there is one fruit people taking statins should be wary of – grapefruit.

Mixing food, drink and some herbal remedies with medications can cause an array of side effects

According to the NHS, a doctor may advise patients taking statins to avoid eating and drinking grapefruit completely. Others may be encouraged to only consume small quantities. 

Dr Leyla Hannbeck, chief executive at the Association of Independent Multiple Pharmacies, said: 'Grapefruit juice should be avoided when taking some cholesterol lowering medicines such as statins because it prevents an enzyme that is needed for the statins to be metabolised in the body.

'As such, it can cause these medicines to accumulate and increases the risk of side effects.'

Statin side effects can include headaches, dizziness, muscle pain and feeling tired, according to the NHS. 

A glass of grapefruit juice could cause aches and pains and even dizziness if you are taking the cholesterol lowering medication statins

Warfarin and leafy greens

Used to prevent and treat blood clots, warfarin has been commonly prescribed in the UK since the 1950s.

However, while we're all being encouraged to eat more leafy greens, like broccoli and spinach, those on the drug should be careful of these vegetables, experts say. 

'Some green vegetables such as kale, broccoli, spinach that are rich in vitamin K can reduce the affects of warfarin,' warned Dr Hannbeck. 

'Warfarin blocks an enzyme that uses vitamin K,' explains Dr Dipa Kamdar pharmacist and lecturer at Kingston University in London. 

'Green leafy vegetables have got quite a high amount of vitamin K in them. This interacts with the warfarin and speeds up its removal from the body.

'This means that the patients that are taking that are at increased risk of strokes.'

If you do want to change your diet you should tell your prescriber so they can adjust your warfarin dose, experts say. 

Eating too many leafy greens, like broccoli and spinach, can prevent warfarin (pictured) from doing it's job

Warfarin and cranberry juice

It's not just leafy greens that warfarin patients need to be wary of.

A refreshing glass of cranberry juice and eating cranberries could also land them in trouble. 

This interaction has the opposite effect to the vitamin K interaction. Instead of stopping the drug from working, it exacerbates its effects.  

As a result, patients can suffer from unexpected or excessive bleeding.

'The reason why pharmacists advise against drinking grape fruit juice if on warfarin is because warfarin thins the blood and grapefruit juice enhances the effect of warfarin leading to increased risk for bleeding,' said Dr Hannbeck. 

St John’s Wort and the contraceptive pill

St Johns Wort is a popular herbal remedy sold over the counter as a treatment for people suffering depression, though the NHS doesn't recommended it for this use.  

It's often assumed to be harmless simply because it is a herbal remedy, but pharmacists warn this isn't always the case.   

St John's Wort is an enzyme inducer, meaning it can speed up the breakdown of other substances. 

One of those substances is the contraceptive pill. 

Mixing St Johns Wort and the contraceptive pill increases the chances that someone taking the contraceptive pill could get pregnant

Dr Kamdar said: 'A lot of people think St John's Wort is harmless because it is a herbal thing, but it speeds up the breakdown of the contraceptive pill in the body. 

'They both work on the same enzyme and it speeds up the rate of this enzyme. Therefore, the concentration of the contraceptive pill in the body is reduced and it might not work properly.'

As a result, mixing the two increases the chances that someone taking the contraceptive pill could get pregnant. 

Pharmacists also warn that, somewhat counterintuitively, St John's Wort can make depression worse if somebody is already taking medication for the mental health condition. 

Dr Hannbeck added: 'St John’s wort also interacts with some antidepressants such as the SSRI, and can increase the risk of side effects and as such should be avoided during the treatment.'

Antidepressants and ibuprofen 

Herbal remedies aren't the only interaction people on antidepressants need to be aware of.  

One type of of antidepressants called selective serotonin reuptake inhibitors (SSRIs) carries a potentially dangerous interaction with anti-inflammatory drugs such as the commonly used over-the-counter painkiller ibuprofen.

Dr Kamdar said: 'SSRIs and anti-inflammatory painkillers, such as ibuprofen, both can increase the risk of bleeding and most commonly in the stomach. 

A certain type of antidepressants called selective serotonin reuptake inhibitors (SSRIs) can interfere with anti-inflammatory drugs such as ibuprofen

'So, taking them together can increase the risk of that.'

Bleeding in the stomach can cause dark stools, stomach cramps, feeling tired and dizzy, according to the NHS.

People taking SSRIs should consult their doctor or a pharmacist before taking anti-inflammatory painkillers. 

Antibiotics and alcohol

Pharmacists will, as a matter of course, encourage people to avoid drinking alcohol while they're taking antibiotics.

But this is particularly highlighted for those taking the antibiotic metronidazole, which is most commonly prescribed following a dental procedure. 

Dr Hannbeck warned mixing the two could cause side effects including vomiting and flushing, known as a 'disulfiram-like reaction'.

The NHS says: 'Do not drink alcohol while you're taking metronidazole, including the two days after you finish. This gives the medicine time to leave your body.

'This is important because metronidazole can react with alcohol to cause a number of side effects. These include feeling and being sick, stomach pain, hot flushes, a pounding heartbeat (palpitations) and a headache.'

Calcium and antibiotics 

Calcium supplements are commonly taken by older people to strengthen bones and prevent fractures.

However, rather than boosting your health, calcium supplements can actively harm it by interacting with several drugs, pharmacists warn. 

These range from antibiotics to drugs used to help prevent people from catching malaria.

Dr Kamdar said: 'These kinds of interactions occur in the stomach and it's to do with the absorption of the medication. 

'So, there are certain things like this calcium that you've got to be careful when using with certain other medication.' 

Antibiotics such as doxycycline, which is often prescribed for acne, are among those affected if you take calcium supplements, pharmacists warn. 

Calcium can interact with several drugs from antibiotics to drugs used to prevent malaria, pharmacists warn

But this isn't just the case with calcium supplements but also with foods rich in the mineral like milk, cheese and even yoghurt.

Dr Hannbeck said drugs belonging to class of antibiotics called tetracyclines, which includes doxycycline, were particularly vulnerable to this interaction. 

'The calcium in dairy products can reduce the effect of these antibiotics by affecting their absorption in body,' she says. 

But Dr Kamdar added there are ways to still have your calcium without affecting your antibiotics. 

'Because this interaction is happening in the stomach it's about keeping a time gap of about an hour,' she says. 

Alcohol and antihistamines

Antihistamines are drugs commonly used to relive symptoms of allergies, such as hay fever or reactions to insect bites.

Some antihistamines such as, chlorphenamine, diphenhydramine and hydroxyzine, make you feel sleepy, according to the NHS.

Side effects of these drugs include reduced co-ordination, reaction speed and judgement as well as drowsiness. 

And mixing them with alcohol can make these symptoms even more extreme, warns Dr Kamdar.

She said: 'You shouldn't be mixing alcohol really at all with these drowsy antihistamines because it can make you very drowsy.'