Vijay Singh has withdrawn from the Phoenix Open, a day after saying he used deer-antler spray.

Singh cited a back injury in pulling out Thursday before the first round.

The 49-year-old Fijian first revealed he used the spray in an interview with Sports Illustrated. The magazine said Singh paid one of Sports With Alternatives To Steroids' owners $9,000 US last November for the spray, hologram chips and other products. The deer-antler substance contains a banned performance-enhancer connected to human growth hormone.

Singh released a statement Wednesday at the Phoenix Open, saying: "While I have used deer-antler spray, at no time was I aware that it may contain a substance that is banned under the PGA Tour Anti-Doping Policy."

PGA Tour spokesman Ty Votaw says the tour is "looking into the matter."

Meanwhile, Mark O'Meara says Vijay Singh should be suspended "for a couple of months" by the U.S. PGA Tour  for his admission.

O'Meara, playing at the Dubai Desert Classic on Thursday, said he was "surprised" to hear about the Fijian's admission but that he doesn't believe it influenced his golfing game. Still, Singh "had to pay the price" for taking a substance prohibited by the tour.