A method of producing cannabinoids for use in medical treatments by growing cultured Cannabis sativa plant cells through tissue culture, the method of comprising the steps of: selecting Cannabis sativa leaf tissue for culture; and growing a tissue culture from the selected leaf tissue in a liquid based medium whilst controlling the light exposure of the tissue culture to control the cannabinoid content of the tissue culture. Control of the light exposure can enable the phytocannabinoid content of the grown tissue culture to be tailored to the use intended for the tissue culture. For example, the THC content of the tissue culture can be controlled to be maximised or minimized depending on the intended use. Use of tissue culture is beneficial as compared to prior art methods as it allows for genetic consistency and reduces the resources necessary to produce plant cells containing phytocannabinoids.