According to MSNBC.com, baby soaps have been linked to false positives for marijuana exposure in infants. The piece explains,

In the study, urine samples that contained minute amounts of any of five baby soaps — Johnson & Johnson’s Head-to-Toe Baby Wash, J&J Bedtime Bath, CVS Night-Time Baby Bath, Aveeno Soothing Relief Creamy Wash and Aveeno Wash Shampoo — gave a positive result on a drug screening test for tetrahydrocannabinol (THC), the active ingredient in marijuana.

The researchers don’t know exactly why the false positives are occurring.  It could be that the chemicals in the soap combine in a way that resembles THC.  Or it could be “that the chemicals change the way the test works,” said one of the researchers according to MSNBC.

As an interesting side note, the story I posted recently of the Butte County parents who lost their children to foster care for 4 months because of the children’s supposed exposure to marijuana, had the DA there explain the reason for not testing the newborn as “insufficient hair to test.” There obviously is some other exposure test widely available for newborns (though apparently easily contaminated by some soaps.) I wonder why that wasn’t used.