The science behind how alcohol and antibiotics interact in the body

But why is that the case? Apart from getting really drunk and forgetting to take your medication that is. Can drinking while taking antibiotics make you feel sicker than you already are?

Does it alter the efficacy of the medication we're taking for better or worse? Does it really matter that much?

Let's distil the science behind how alcohol and antibiotics interact in the body.

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Broadly, alcohol is a class of organic compounds that have one or more hydroxyl groups attached to a hydrocarbon chain. Those are some weird chemistry terms, but a hydrocarbon chain contains just carbon and hydrogen atoms - think of the carbon atoms as the metal links of a bracelet, with the hydrogen atoms branching off them like charms.

A hydroxyl group is a hydrogen atom bonded to an oxygen atom, which is bonded to a carbon atom of the hydrocarbon chain.

Alcohol is used in solvents, hand sanitisers, and cosmetics but the specific alcohol found in the drinks we consume is called ethanol - or ethyl alcohol - which means the hydrocarbon chain contains only two carbons. Ethanol is made when sugars, usually sourced from grains, fruits and vegetables, are fermented by single-celled organisms called yeast.

When alcohol is ingested, a small amount of it is immediately broken down or metabolised in your stomach into acetaldehyde. But most of the remaining alcohol is absorbed into the bloodstream, through the stomach and upper small intestine, and makes its way to the liver for the first time.

Once it arrives, only a small amount of the alcohol is metabolised, while the rest leaves the liver, enters general circulation, and is distributed through the tissues of the body. This is called the "first pass metabolism".

Then, the remaining alcohol returns to your liver to be metabolised for the second and final time by several enzymes, which are proteins that catalyse biochemical reactions, the most important of which here are alcohol dehydrogenase and Cytochrome P450.

These and other enzymes downstream, break the alcohol down until in the end, carbon dioxide and water leave your body.

Antibiotics are substances used to treat bacterial infections by either killing bacteria or inhibiting them from growing. Inside the body, antibiotics are either eliminated in their active form through the kidney or are metabolised by the liver.

A 2020 review of all the scientific evidence available behind alcohol and antibiotic interactions combed through 87 studies to determine the evidence behind the alcohol warnings issued for many common antibiotics. But they found that alcohol consumption shouldn't cause problems when taking a number of different common antibiotics.

The available data supports that certain, specific antibiotics, and more broadly a couple of select classes of antibiotics, show no adverse interactions when taken with alcohol.

For example: oral penicillins, the antibiotics cefdinir and cefpodoxime (from the class of antibiotics called cephalosporins), the broad spectrum class of antibiotics called fluoroquinolones, and the antibiotics azithromycin and tetracycline.

Alcohol can interact with medications in two different ways. Pharmacokinetic interactions occur when alcohol interferes with the metabolism of a medication, or the medication interferes with the metabolism of the alcohol.

This generally happens in the liver where both alcohol and some antibiotics are metabolised - even sometimes by the same enzymes.

An example of this is the frontline anti-tuberculosis antibiotic isoniazid. On its own, isoniazid can cause acute liver injury and alcohol can exacerbate this.

In people who consume alcohol occasionally a component of Cytochrome P450 only metabolises a small fraction of the alcohol ingested, but its activity can be increased ten-fold in chronic heavy drinkers that consume 3 to 4 standard drinks per day.

When sober this enhanced activity can increase the rate of the breakdown of the antibiotic, and when intoxicated alcohol competes with the antibiotic to be metabolised and the antibiotic's breakdown is slowed.

The second kind of interactions are pharmacodynamic, where alcohol enhances the effects of a medication or vice-versa.

The antibiotic erythromycin is used to treat a wide variety of bacterial infections. But it also increases gastric contractions and accelerates the emptying of the stomach, which means it can reduce the first-pass metabolism of alcohol and lead to faster alcohol absorption in the small intestine.

In fact, peak alcohol concentrations have been found to be higher after taking erythromycin, increasing by 40% on average. Unfortunately, alcohol also decreases the efficacy of erythromycin.

There are also antibiotics that can unexpectedly trigger a disulfiram-like reaction when mixed with alcohol.

Disulfiram is a treatment for heavy alcohol use. It inhibits an enzyme important to alcohol metabolism, which causes the rapid accumulation of a toxic by-product of alcohol metabolism in the blood. This causes unpleasant effects including facial flushing, nausea, headaches, vomiting, chest pain, vertigo, sweating, thirst, blurred vision, and low blood pressure.

Unfortunately, the reaction occurs with uncertain frequency and with varied severity across numerous antibiotics - so it's not as simple as steering clear of one or more broad classes of antibiotics.

At the end of the day, as with all medications, it's important to talk openly with your own healthcare professional about the potential effects of mixing alcohol with antibiotics on you.