If it is plastic I would change it out and replace it with a brass drain valve. Some may call it a boiler drain valve. Depending on condition of the valve you currently have will determine if it is repairable.
You have to drain it down to repair and it would just be as easy to replace it since you drained it.
Most hardware stores should carry this valve
Below is a link with some information about upgrading the drain on the heater to avoid problems.
http://ezinearticles.com/?Avoid-Leaks-At-The-Bottom-Of-Your-Water-Heater:-Upgrade-Your-Drain-Valve&id=267639
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