Asda is recalling all corned beef from its budget range after traces of the veterinary drug phenylbutazone were found in some batches.
The Food Standards Agency said "very low levels" had been detected in the Asda Smart Price Corned Beef product.
The painkilling medicine often used on horses is commonly known as "bute".
The Food Standards Agency (FSA) said it was the first time bute had been found in a meat product in the UK since the horsemeat scandal started.
Animals treated with bute are not allowed to enter the food chain because the drug could pose a risk to human health.
However, the risk is very low even if people have eaten contaminated horsemeat.
The Asda product was tested as part of an industry-wide programme and found to be positive for horse DNA above 1%.
It was then further tested and found to contain four parts per billion of bute.
Asda had already withdrawn the product on 8 March.
Customers who have bought the 340g tins with any date code have been urged not to eat the corned beef and to return it to the supermarket for a refund.
Chief medical officer Professor Dame Sally Davies said: "Horsemeat containing phenylbutazone presents a very low risk to human health.
'Phenylbutazone, known as bute, is a commonly used medicine in horses. It is also prescribed to some patients who are suffering from a severe form of arthritis.
"In patients who have been taking phenylbutazone as a medicine there can be serious side effects, but these are rare. It is extremely unlikely that anyone who has eaten horse meat containing bute will experience one of these side effects."
In the UK, horse carcasses must have a negative bute test before they are allowed to enter the food chain.
Earlier, the FSA said two beefburger products withdrawn from sale over concerns they may contain horsemeat had tested positive.
The King Fry Meat Products burger was from Pig Out in Walsall and the Burger Manufacturing Company product was from Nefyn Pizza and Kebab House in Gwynedd.
Five samples were checked for the presence of horse DNA above a 1% threshold.
Two samples did not contain horse DNA and one result is still to be returned.