In 1975, Dr. Marc Kurzman at the University of Minnesota, in collaboration with fourteen other scientists, published a study in The Journal of Criminal Defense that concluded: "The microscopic and chemical screening tests presently used in marijuana analysis are not specific even in combination for 'marijuana' defined in any way.
However, many plant species have such hairs, and the study only confirmed that 82 of them did not give D–L test false positives.
The study also noted that "mace, nutmeg and tea reacted with the modified Duquenois–Levine [test]".
A 2012 Brazilian study tested 40 vegetal drugs with the Duquenois–Levine test and obtained false positive results from Chilean boldo (Peumus boldus Molina), pot marigold (Calendula officinalis L.), leather hat (Echinodorus grandiflorus (Cham.
), lemon balm (Melissa officinalis), anise (Pimpinella anisum L.), guaraná (Paulinia cupana Kunth.
Twenty drops of chloroform (or similar solvent) are then added, and the tube is vortexed, then allowed to settle and separate into two layers.
Marijuana (as well as a variety of other plant substances) becomes purple with the addition of the Duquenois reagent and hydrochloric acid.