The Ministry of Defence spent £4.2m on anthrax vaccine last year, government figures show.
Anthrax is highly contagious
In a written statement, defence minister Don Touhig also revealed the shelf life of the vaccine had been extended from two to three years.
He said the MoD held enough in stock to "meet its projected requirements" but would not say how much this was.
"The precise numbers held cannot be released as this would be prejudicial to defence activities," he added.
The statement came in response to a question from Tory MP Andrew Robathan.