United Health is down 3.76% at $157.50 a share on Friday morning after news that the US Justice Department joined a 2011 lawsuit by a former United Health executive against the health insurer. 

The lawsuit claims that United Health overcharged Medicare by hundreds of millions of dollars by claiming the program's participants were sicker than they actually were, according to the law firm Constantine Cannon LLP.

Constantine Cannon posted the lawsuit online when it was unsealed on Thursday.

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