Why a hot bath could turn your daily pills into a killer dose

When it comes to taking medication, most of us are aware that certain conditions can alter their effectiveness.

For instance, some pills should be taken with or after food, while some are best swallowed on an empty stomach.

But there are other surprising factors that can also influence how a drug works — and some may have fatal consequences.

Be aware that any medication exposed to hot temperatures might cause physical changes that mean they lose potency

Be aware that any medication exposed to hot temperatures might cause physical changes that mean they lose potency

Reports this month of the death of Barbara Reynolds, 67, highlights the need to check how we take medication.

The heat of a bath she was taking sped up the absorption of her painkilling fentanyl patches (when she put on a new patch without removing the old), causing a surge of the drug that stopped her heart.

Dr Tim Johnson, a consultant in pain management, says: ‘The effect of the heat on the patches would provide a surge of fentanyl which would anaesthetise you.’

Professor Simon Maxwell, a clinical pharmacologist at the University of Edinburgh and a member of the British Pharmacological Society, says: ‘Many factors can influence the successful treatment of someone on medications.’

Here, with Professor Maxwell’s help, we list what to look out for to ensure medications work safely and effectively.


Medicines absorbed through the skin via patches are done so more quickly in the heat because the blood vessels dilate, increasing blood flow and speeding up the transport of the drug away from the skin and into the body.

This is known to be a risk with painkillers taken this way, such as fentanyl, oxycodone and buprenorphine. So be aware of hot baths and other hot conditions, such as sunny days.

‘This is theoretically true for any patch — from nicotine to HRT patches,’ says Professor Maxwell.

Be aware that any medication exposed to hot temperatures might cause physical changes that mean they lose potency.

Freezing temperatures can cause similar problems. Drugs such as insulin might lose their effectiveness.

‘Freezing destroys the insulin molecules,’ says Professor Maxwell. ‘This could, in theory, stop the drug being absorbed as quickly.’

Some pills have special coatings that affect how they are released

Some pills have special coatings that affect how they are released

Crushing pills to make them easier to swallow can cause serious — even fatal — side-effects.

Some pills have special coatings that affect how they are released (so they act slowly or regularly).

‘Drugs with a special coating should not be crushed or chewed,’ says Professor Maxwell.

‘This only increases its surface area and so its speed of absorption.’

Specially coated drugs that should not be crushed include morphine, as it could lead to a fatally fast release of the drug.

Nifedipine, used for angina and blood pressure, can cause dizziness, headaches and an increased risk of stroke or heart attack when crushed.

Never take tablets that stick together, are chipped, or have changed colour or consistency, as they may have been affected.

‘Check expiry dates on medication, too,’ adds Professor Maxwell.


Cranberries and its juice can interact with the blood-thinning medication warfarin, given to patients with atrial fibrillation (irregular heart beat).

Cranberries contain large amounts of salicylic acid, present in aspirin, which also thins the blood.

‘Excess blood-thinning can lead to life-threatening bleeding,’ says Professor Maxwell.

Meanwhile, grapefruit and some other citrus fruits such as Seville oranges can interfere with enzymes that break down some medications in the digestive system.

Naringin, naringenin and bergapten — compounds found in citrus fruits — can knock out an enzyme in the gut that usually partially destroys a drug as it is absorbed, transforming a safe medication into a potential overdose.

‘More medication stays in the body, which can increase their strength to dangerous levels,’ says Professor Maxwell, adding that some people have died as a result.

Grapefruit can also affect medications such as high blood pressure drugs and statins, and some antihistamine treatments.

Orange and apple juice can combine badly with tablets such as the anti-cancer drug etoposide, beta blocker atenolol, celiprolol and talinolol, and antibiotics ciprofloxacin, levo-floxacin and itraconazole.

Experts stress that eating a few segments of the fruit is thought to be safe; it is the concentration of the chemicals in the juice that causes adverse reactions. Speak to your GP if unsure.


Drinking alcohol regularly speeds up the rate at which our liver breaks down compounds — but this also means it processes some drugs more swiftly.

This means the concentration of a medicine in the body is lower than expected, so may not work as well.

Alcohol depresses the nervous system, tranquillising the effect from medications

Alcohol depresses the nervous system, tranquillising the effect from medications

‘It’s difficult to say how much alcohol will affect things, as we all differ, but as little as two glasses per day could increase the rate at which drugs are metabolised,’ says Professor Maxwell.

‘Warfarin used to treat atrial fibrillation is the best example of a drug prone to this.’

Alcohol also depresses the nervous system, tranquillising the effect from medications. This means the sedation from drugs such as diazepam (Valium), prescribed to treat anxiety, may cause drowsiness and make accidents or falls more likely.

Smoking can have a similar effect on medication. However, its effect is less noticeable.


Age has a major effect on medication.

‘Due to the ageing process our kidney function deteriorates,’ says Professor Maxwell.

‘At 70, our kidneys function two-thirds less efficiently than at 20. As drugs are removed from the body by excretion in urine, medication doses need to be frequently assessed to ensure the right one is being taken — otherwise too much of a drug can build up.’

If someone is taking a medication long-term, the dose will need to be considered at regular intervals.

For example digoxin — which slows heart rate and strengthens heart contractions in people with atrial fibrillation or is used to treat heart failure — can slow the heart, causing nausea and faintness, if taken in excess.


‘Harmful interactions when more than one type of medication is taken at the same time are common and sometimes lead to admission to hospital,’ says Professor Maxwell.

One medication can change the way another is carried in the blood. This may have an impact on how effective a particular medication is at fighting the condition for which it was prescribed.

Also, the way in which one medication is broken down by the body can be altered by another drug.

If medicines that can have similar side-effects are taken together, there may also be a chance that the side-effects will be additive.

One of the main problems seen is with warfarin, as its effect can be increased by some antibiotics.

Also, some drugs for acid reflux, called cimetidine, which reduces acid production in the stomach, can also cause thinning of the blood. When taken with more blood–thinner warfarin, it can lead to bruises and bleeding.

In rare cases, this could be fatal, as it reduces the ability for the blood to clot and so stop bleeding.