UPDATE 1-US warns Allergan on acne drug promotion

* Aczone ad overstated effectiveness, FDA says

* Company says ad no longer in use

* Allergan shares down slightly

WASHINGTON, Aug 21 (Reuters) - An Allergan Inc (AGN.N) advertisement for acne drug Aczone "grossly overstates" the drug's effectiveness, U.S. regulators said in a letter released on Friday.

The Food and Drug Administration also said the medical journal ad left out "important risk information" and "misleadingly suggests that Aczone is safer than has been demonstrated," the Food and Drug Administration said.

Allergan spokeswoman Caroline Van Hove said the ad ran in March, April and May and is no longer in use. A new ad "which addresses the key concerns in the FDA letter" started running in May, she said.

Allergan shares fell 11 cents to $55.46 in afternoon trading on the New York Stock Exchange.

The FDA posted the letter here

(Reporting by Lisa Richwine, Editing by Leslie Gevirtz)