Effects of food, drink & smoking on medication

 In Clinical Guidelines, Feature article, Medicines Management Processes

The effect of medicines can be influenced by many factors such as the patient’s age, sex, or weight or interactions with other drugs. Often over-looked are the effects of food, drink or smoking which can have significant adverse interactions with drugs. This usually results from changes in drug blood levels caused by the way a drug is absorbed, distributed, metabolised or excreted which are otherwise known as pharmacokinetic effects.

Food can delay absorption of many drugs, such benzodiazepines, so there could be a delayed response to oral treatment following a meal. This is why many medicines are labelled with the instruction “Take on an empty stomach.” In other cases, it is recommended that medication should be taken after food and this may be because food can protect the stomach from an irritant drug.

Hydration will affect the distribution of drugs, and this is particularly significant for lithium. Dehydration could result in toxicity caused by higher serum lithium levels.

Alcohol acts as a drug and has central depressant effects which can act similarly and cumulatively with many psychotropic drugs, such as benzodiazepines, causing drowsiness and reducing the ability to perform skill-based tasks.

Grapefruit inhibits metabolism by liver (CYP450) enzymes which can increase levels of drugs such as sertraline, and midazolam and is best avoided with some drugs such as the statins simvastatin and atorvastatin.  The BNF (Appendix 1) has a section on drugs affected by grapefruit.

Smoking reduces clozapine blood levels due to the effect of enzyme-inducing chemicals in smoke. If a patient stops smoking, clozapine levels can increase by 72%, causing toxicity. This will also happen if the patient switches to an electronic cigarette or has nicotine replacement therapy because it is the cigarette smoke that is significant, not the nicotine. Also, clozapine metabolism is inhibited by caffeine and stopping caffeine consumption can reduce clozapine levels by 50%.

The antidepressants known as monoamine oxidase inhibitors (MAOIs), work by inhibiting the enzyme monoamine oxidase which is responsible for breaking down monoamine neurotransmitters, including serotonin, in the brain.

Under normal circumstances monoamine oxidase present in the liver and gut wall breaks down the dietary monoamine tyramine prior to it reaching the systemic circulation. However, if this is inhibited the result can be adverse effects including hypertension and hypertensive crisis which can be fatal.

The key advice for patients is to avoid foods rich in tyramine such as certain cheeses (especially mature), broad beans, pickled herring, chicken liver pate, aged and cured meats, soy, soybean and yeast or meat extracts. Drinks such as red wine or real ale may also be problematic.

The above only provides a few examples of food, drink and medicine interactions and not all such interactions are highlighted in the BNF. If you suspect food or drink related drug interactions are leading to adverse effects or treatment failure, you can contact your visiting pharmacist for advice.

A good starting point to check food and drug interactions is the BNF cautionary and advisory labels. BNF monographs have label numbers written at the end of each entry which relate to a list on the back page of the BNF. For example, BNF Label 21 states “Take with or just after food, or a meal.’ Such advisory statements are generally drawn from the Summary of Product Characteristics which is another useful resource should you need more detail.

Other good reference sources include the Choice and Medication website, especially when providing information to patients.

Choice and Medication is available via Ashtons Hospital Pharmacy Services online at:

In addition, Ashtons medical information has access to Stockley’s Drug Interactions and other resources, so contact your visiting pharmacist for further advice.

A summary of product characteristics is available via:
Written by:

John Baylis,
Clinical Pharmacist

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