Better known as bute, the drug was used in the 1950s to treat arthritis and gout in humans, but has since been withdrawn from pharmacy shelves.
"It was found to be dangerous for humans, so for decades it has only been used as a veterinary drug," said World Health Organisation food safety expert Peter Ben Embarek.
The United States’ Food and Drug Administration has banned the drug’s use in food-producing animals, including dairy cows.
In Europe, a law requires a passport for every horse to declare whether or not it is destined for the abattoir — in which case it may not consume any on a long list of prohibited medicines that include bute.
The drug today is used a painkiller and anti-inflammatory drug in dogs and horses that are not meant to be eaten.
It is an inexpensive and effective treatment for sore joints and to aid recovery after fractures.
Bute can be injected or fed to the horse as a powder or paste, often mixed with their feed.
Online product information for one veterinary brand of phenylbutazone granules warns of side-effects including ulcers, kidney damage and internal bleeding, especially in young, ill or stressed horses.
It urged cautious use in pregnant or nursing mares as it may be toxic to the foal.
The pamphlet also urged extreme caution for people handling the drug, with as little contact as possible, and warns to "never breathe powder from crushing tablets".
In humans, phenylbutazone can induce blood disorders like aplastic anaemia, which causes the bone marrow to stop producing enough red and white blood cells and platelets.
People with severe forms of the disease are at risk of life-threatening infections or bleeding.
These adverse reactions were, however, associated with human use of 200 to 800 milligrams of phenylbutazone per day, according to a fact sheet issued by the Science Media Centre in London.
Broken down, bute is detectable in horse flesh, "but at concentrations considered far too low to be of concern," it added.
"Bute is known to be a carcinogen (cancer-forming) in rats, but there is no conclusive evidence for it to be carcinogenic in humans."
The centre added that about 8,000 to 10,000 horses are slaughtered for human consumption in the United Kingdom every year.
In the past five years, between two and five percent of samples taken at abattoirs have tested positive for bute in the United Kingdom.
"The idea that you might get a clinically significant amount in horse meat even after therapeutic administration to the horse is, frankly, daft," said Colin Berry, a retired University of London pathology professor.
Horsemeat is widely eaten in parts of Asia and Europe, including in France, but is considered taboo in Britain.
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