The active chemical in cannabis is THC (Delta-9 tetrahydrocannabinol). The more THC cannabis contains, the stronger it is. THC is absorbed into the bloodstream through the walls of the lungs (if cannabis is smoked), or through the walls of the stomach and intestines (if eaten). The bloodstream carries the THC to the brain, producing the ‘high' effects. Drugs inhaled get into the bloodstream quicker than those eaten leading to a more rapid effect.
Some research has indicated that the average THC content in cannabis has risen considerably over recent year, mainly due to the development of indoor hydroponic growing techniques (which means that cannabis is now freely available year round) and different strains of the cannabis plant being cross-bred.