NEW YORK – Deaths from heroin overdose doubled in just two years in much of the nation, a new government study says, between 2010 and 2012 in the 28 states for which information was available to the researchers.

Overdose numbers from all the states are not expected to be released for at least a few more months.

The annual number of U.S. drug overdose deaths has been growing for more than 20 years.

Experts believe one reason heroin-related deaths increased may be because people who had been abusing the painkillers may have switched to heroin.

Recent restrictions on prescribing opioid painkillers may be reducing illicit supplies of them at a time when the heroin supply has been increasing, CDC officials said.