Authentic hair samples from Cannabis users and a drug free hair sample which was separately spiked with tetrahydrocannabinol (THC), cannabidiol (CBD) or cannabinol (CBN) were exposed outside as well as to natural sunlight at prevailing and elevated humidity in quartz glass tubes during 8 weeks. In addition, authentic and spiked hair samples were exposed to xenon arc radiation in a light exposure cabinet for 24 hours. Stability of THC, CBD and CBN in authentic samples differed from that of the spiked hair. The radiation experiment revealed that CBN could not be measured in hair which had been spiked with THC. Under all conditions chosen the concentrations of THC, CBD and CBN decreased. At high humidity the concentrations declined more rapidly. In both authentic and spiked samples THC was most unstable compared to CBD and CBN. Therefore, in hair analysis determination of CBD and CBN seems promising to detect Cannabis exposure even under unfavorable conditions.