Manu Khajuria and Dhiraj Vyas Plant Sciences & Agro technology Division, Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu, J & K 180001 India. E-mail addresses: manu.khajuria@iiim.res.in Cannabis sativa L is a source of medicinally important phytocannabinoids including tetrahydrocannabinol (THC) and cannabidiol (CBD). Light is a key modulator of biomass and cannabinoid yield which suggests responsive photochemical machinery. In the present study the effect of the increasing levels of metabolic THC content on the photochemical efficiency was studied in Cannabis. The chlorophyll a fluorescence kinetics, immunodetection of the photosynthetic machinery and photosynthetic pigments was analyzed on seven different accessions from different environments. Based on their relative content of CBD and THC, the accessions were clearly divided into three groups. Group I (CBD/THC > 1) showed clear benefit in terms of the damage to the RbCL, D1 and Lhc1 holo-complex protein. OJIP kinetics also suggested a negative correlation with the THC content. In Group III with high THC content (THC > 6%), zeaxanthin-dependent quenching is primarily responsible for lower NPQ. Further, THC treatment on Arabidopsis thaliana also suggested a dose-dependent decrease in the photochemical efficiency. The exclusivity of THC in causing the response resulted in the damage of photosynthetic machinery and the generation of free radicals, thereby compromising the yield. This study helps in opening a new screening method for Cannabis, based on photochemical efficiency and cannabinoid content. Keywords: Cannabidiol, Cannabis sativa, Non-photochemical quenching, OJIP Kinetics, Photochemical efficiency, Tetrahydrocannabinol