(AP) -- Authorities are warning residents in southern New Jersey that Camden appears to be the center of a spike in recent heroin overdoses.

The Courier-Post reports authorities in the city responded to at least 29 heroin overdoses between Tuesday and Thursday. An alert from the state police indicates that many of the overdoses occurred when people used free samples of heroin labeled with a Batman stamp.

The alert says none of the overdoses were fatal.

State police say the Batman heroin can be so dangerous that emergency personnel may need extra Narcan to reverse overdoses and revive patients. Authorities believe the drug has been sold over the past two weeks in Camden.

But officials say the problem has spread to other parts of the state.