Singh pulls out of Phoenix Open
Vijay Singh pulled out of the Waste Management Phoenix Open before the start after admitting he had used a deer antler spray.
Singh admitted he had used the spray which has previously been found to contain a banned growth hormone.
The 49-year-old Fijian, now facing the possibility of a worldwide suspension, withdrew after issuing a statement in which he said: "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.
"In fact, when I first received the product I reviewed the list of ingredients and did not see any prohibited substance.
"I am absolutely shocked that deer antler spray may contain a banned substance and am angry that I have put myself in this position.
"I have been in contact with the PGA Tour and am co-operating fully with their review of this matter. I will not be commenting further at this time."
Singh, currently ranked 89th in the world, was quoted in a Sports Illustrated investigation as saying he used the spray "every couple of hours... every day" and was "looking forward to some change in my body".
Two years ago 1999 Open champion Mark Calcavecchia was advised to stop using such a spray and PGA Tour spokeswoman Laura Hill said at the time: "The PGA Tour regularly warns players of the risks associated with all supplements."
Deer antler spray is manufactured by Sports with Alternatives to Steroids (SWATS) and it has been found to contain the banned IGF-1, a natural anabolic hormone that stimulates muscle growth.
Scientific research has shown that velvet deer antler improves heart strength, stamina, joint health, muscle and strength development plus athletic performance.
Christopher Key, of SWATS, said in the Sports Illustrated article: "The antlers are the fastest-growing substance on planet Earth... because of the high concentration of IGF-1.
"We've been able to freeze dry that out, extract it, put it in a sublingual spray that you shake for 20 seconds and then spray three (times) under your tongue. This stuff has been around for almost 1,000 years."
Since drug testing was introduced in golf in 2008 only one PGA Tour member has been banned for violating the code.
American Doug Barron received a one-year suspension after failing a drugs test at the St Jude Classic in 2009.
He was taking Lyrica as a substitute for propranolol, a banned substance and exogenous testosterone, which he received by an injection from a doctor.
Singh spent 32 weeks as world number one between 2004 and 2005, won the US Masters in 2000 and the USPGA Championship in 1998 and 2004.
Earlier in his career, however, he served a ban in Asia over a scorecard incident.
Meanwhile, New Zealand's former Open winner Bob Charles said today he used and promoted the antler spray for more than 20 years and was surprised to learn it contained a banned substance.
The PGA Tour, which has a policy of not announcing any disciplinary action, reported on its website that Singh cited a back injury for his withdrawal from this week's event.
He was 27th in last week's Farmers Insurance Open at Torrey Pines and, in his only other start this season, was 20th at the Sony Open in Hawaii.