The veterinary medicine was found in 340g tins of its Smart Price Corned Beef in levels of four parts per billion (ppb) and is the only meat product where bute has been found so far.
Asda has also recalled its Chosen By You Corned Beef (340g) product, also withdrawn in March because it is produced in the same factory as the affected product.
Animals treated with bute should not enter the food chain as the drug, which is banned, may pose a risk to human health.
Low risk to human health
Both the UK Food Standards Agency (FSA) and Asda stressed the low risk to human health.
The product was tested by Asda as part of the industry testing programme and found to be positive for horse DNA above 1% and was subsequently withdrawn on 8 March.
Professor Dame Sally Davies, chief medical officer, Sally Davies previously said horse meat containing phenylbutazone presents a very low risk to human health.
“The levels of bute that have previously been found in horse carcasses mean that a person would have to eat 500 - 600 one hundred per cent horsemeat burgers a day to get close to consuming a human's daily dose.
“And it passes through the system fairly quickly, so it is unlikely to build up in our bodies.”
No other Asda products are known to be affected and the FSA said it was investigating the issue.
In a statement Asda said it wanted customers to have complete confidence in its food and would offer refunds.
“We have taken an extremely cautious approach since the very beginning and have carried out more than 700 tests, moving swiftly to remove any products from our shelves whenever we’ve had the smallest concerns.
“The tinned Chosen By You Corned Beef (340g) product, also withdrawn in March, has not tested positive for phenylbutazone. However as a precaution it is also being recalled as it is made in the same factory.”
The FSA previously reported that eight horses slaughtered in the UK between 30 January and 7 February tested positive for bute and six entered the food chain after being sent to France.