* Indexes down: Dow 1.86 pct, S&P 1.83 pct, Nasdaq 1.57 pct (Updates prices)
By Sinead Carew
NEW YORK, Jan 5 (Reuters) - The S&P 500 had its worst day in almost three months on Monday, with energy shares leading the decline as global economic concerns were compounded by swooning oil prices.
The S&P closed off 1.8 percent in its first four-day losing streak since December 2013 and its biggest drop since Oct. 9. It fell as much as 1.99 percent in the session while the Dow Jones Industrial Average tumbled as much as 2 percent.
Crude oil futures prices dropped to their lowest since 2009 amid a global supply glut and lackluster demand. U.S. crude fell 5 percent to below $50 a barrel.
The euro fell to a near nine-year low against the dollar of $1.18605 on the EBS platform, tumbling on political uncertainty in Greece and fears of a possible Greek exit from the euro zone.
The Greece question was front and center for Paul Nolte, portfolio manager at Kingsview Asset Management.
"What you've done is thrown that entire area up for grabs, it will be very hard to see much in the way of economic growth if Greece has to go back to their currency," he said. "It would create a lot more chaos and take everybody's focus away from economic growth."
There were more than three times as many declining stocks on the New York Stock Exchange as advancing issues and more than twice as many Nasdaq losers than gainers.
Strength in the dollar also put more pressure on dollar-denominated commodities. A measure of the greenback against a basket of major currencies hit its highest since December 2005.
The Dow Jones industrial average fell 331.34 points, or 1.86 percent, to 17,501.65, the S&P 500 lost 37.62 points, or 1.83 percent, to 2,020.58 and the Nasdaq Composite dropped 74.24 points, or 1.57 percent, to 4,652.57.
The S&P 500 energy sector closed off 4 percent, its largest daily percentage decline since November 28. It fell almost 20 percent in the second half of last year.
The health sector, the S&P's best performer on Monday, still fell 0.6 percent. Medical device maker Boston Scientific was the S&P's biggest winner with a 4 pct gain after a brokerage upgrade.
Another bright spot was Gilead Sciences Inc whose shares rose percent after CVS Health Corp, one of the largest U.S. managers of drug benefits, said it will give Gilead's hepatitis C treatment preferred status and cover competitor AbbVie Inc's treatment only as an exception. AbbVie shares lost 2 percent.
Declining issues outnumbered advancing ones on the NYSE by 2,413 to 714, for a 3.38-to-1 ratio on the downside; on the Nasdaq, 1,961 issues fell and 801 advanced for a 2.45-to-1 ratio favoring decliners.
The benchmark S&P 500 index was posting 6 new 52-week highs and 6 new lows; the Nasdaq Composite was recording 54 new highs and 44 new lows.
About 7.09 billion shares traded on U.S. exchanges, above the 6.87 billion December average, according to data from BATS Global Markets.
Additional reporting by Rodrigo Campos, Chuck Mikolajczak and Ryan Vlastelica; Editing by Bernadette Baum, Nick Zieminski and Meredith Mazzilli