A Woburn woman has admitted she switched Flintstones vitamins with candy in stores, saying she was angry that vitamins she bought had been tampered with.

Denise Shannon told federal investigators she substituted the chewable vitamins with red-hot candy, jelly beans, M&Ms and gumballs eight to 10 times in 2004 and 2005 at various supermarkets in the Woburn area.

According to a plea agreement, the 38-year-old Shannon said she did it because she felt "violated" after she bought Flintstones vitamins that were tampered with in 2004.

Shannon was indicted on federal charges of tainting consumer products with intent to cause serious injury to a business.

Her plea agreement has not yet been accepted by a judge. She faces up to three years in prison.