Tabloid headlines about the horsemeat scandal

Phenylbutazone (Bute)

The controversial horse painkiller

Paul May
University of Bristol

Also available: JMol version.

phenylbutazone - click for 3D structure

Bute? That's the chemical in the horsemeat scandal isn't it?

Yes, that's right. Bute (proper name phenylbutazone) is an anti-inflammatory drug that is commonly given to horses to treat sore joints and aid recovery after bone fractures.

HorseWhat's the problem with it?

Because it has been linked with a number of adverse side-effects in humans, it isn't licensed for use in animals that then go into the human food-chain.

Why, what problems are there?

It acts in a way similar to other anti-inflammatory drugs, such as aspirin, and so the side-effects are similar too. In humans, its use can cause stomach ulcers, kidney damage, and internal bleeding. Another factor is that when metobolised by the body (or a human or a horse), its metabolites can cause aplastic anemia. This means that if a person eats meat from a horse that had been given bute - even several days before it went to the abatoir - its metabolites may still be present in the meat, and have potential to cause harm.

That sounds serious...

Is it - potentially. Luckily, so far, no traces of bute (or its metabolites) have been found in any of the adulterated meat products that caused the recent food scandal in the UK and Europe. Also, the amounts of bute or metabolites that are likely to make it onto the meat on your dinner plate are so low as to be (almost) negligible. And it's also not believed to be carcinogenic. So, as usual, the risks are probably overhyped to make sensationist tabloid headlines.

various beef products adulterate with horsemeat
In February 2013 various processed beef products on sale in UK
and European supermarkets were found to contain traces of horsemeat.

What about legal horsemeat?

Horsemeat that is sold legally for human consumption in countries such as France and Italy is regularly tested by the EU to make sure it doesn't contain bute or other toxic substances - although with the continuing economic crisis in Europe these tests have been cut considerably recently. In some countries, such as Portugal, food testing has been cut so much that it is almost non-existant, leading to worries that adulterated food could be widespread in these places with no-one knowing about it.

ScrabbleWhat are the metabolites?

The major one is called oxyphenbutazone, which is itself an anti-inflammatory drug with a similar structure to bute - in fact it differs by just one hydroxyl group. It has a claim to fame in that the word 'oxyphenbutazone' holds the title for the highest possible single-play score for Scrabble (American tournament rules), scoring 1,780 points across three triple-word-score squares, joining 7 tiles to 8 already played tiles.

Oxyphenbutazone
Oxyphenbutazone

Dancer's ImageWhy isn't it toxic for horses?

It is, in high doses and with prolonged exposure. Horses are larger animals and so can tolerate larger doses than humans, and also have a wider margin of safety, making it harder to accidentally administer an overdose. So it's reasonably safe to treat horses with it, but not humans or other animals such as dogs. However, it is still not allowed to treat racehorses with bute for a few days before a race, as giving a horse a painkiller might improve its performance.

So racehorses are dope tested for bute?

Yes, and for other banned substances. The most famous case was in America, at the 1968 Kentucky Derby, when the winner - a horse called Dancer's Image (photo right) - was disqualified after a urine analysis found traces of bute. This made US national news at the time and remains one of the greatest scandals in US racing history.

How is bute made

In a lactamisation process involving the condensation of diethyl n-butylmalonate with hydrazobenzene in the presence of base. After work-up and purification, bute is obtained as pure crystals.

Hydrazobenzene + Diethyl-n-butylmalonate Bute + H2O

Hydrazobenzene

Diethyl-n-butylmalonate

Phenylbutazone

References

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