Christine Braun of Jefferson County said her daughter received birthday gifts by mail from a San Clemente, Calif., relative, with another package addressed to the girl, also mailed from San Clemente, arriving a few days later.
The second package contained pastries and other treats, and Braun said after she ate a muffin and her daughter ate a star-shaped candy, both began feeling ill.
"I could not feel my body. I kept having sensations that my blood was really thick and the skin was going to break," she said after the two were taken to St. Anthony's Medical Center in St. Louis.
Doctors said Braun and her daughter tested positive for highly concentrated levels of THC, the active ingredient in marijuana.
The package and the remainder of its contents were turned over to police, KTVI-TV, St. Louis, reported.
"The fact that it was addressed to my 4-year-old daughter is what scares me," Braun said after she and her daughter recovered. "I don't want this to happen to somebody else."