For the study, the researchers selected popular medicinal herbs, and then randomly bought different brands of those products from stores and outlets in Canada and the United States. To avoid singling out any company, they did not disclose any product names. Among their findings were bottles of echinacea supplements, used by millions of Americans to prevent and treat colds, that contained ground up bitter weed, Parthenium hysterophorus, an invasive plant found in India and Australia that has been linked to rashes, nausea and flatulence.