Neither the non-psychotomimetic cannabidiol (CBD) nor the psychotomimetic delta-9-tetrahydrocannabinol (THC) are universally efficacious in reducing cancer cell viability.

In vitro, pure CBD is very often equally or more efficacious than CBD extracts, whereas pure THC is frequently less efficacious than THC-rich extracts.

Although cannabinoids have been shown to slow down tumour growth and/or extend survival in animals, similar observations in man are currently only supported by two pilot studies and a limited number of case reports.