The most important psychoactive component in the cannabis plant, the Delta9- tetrahydrocannabinol (Delta 9-THC), is present in all parts of both the male and female plant. The woody stems and the seeds contain very low concentrations of the substance; the concentration in the leaves is somewhat higher. The highest concentrations are found in the female flower heads, in particular in the resinous glands found inside the flowers. Because hashish contains a lot of resin and is the processing product of highly concentrated ingredients, it contains a relatively high amount of the psychoactive Δ9-tetrahydrocannabinol.
The THC concentration in a plant is dependent on the manner in which it was grown. The THC level in a cannabis plant depends strongly on environmental factors such as temperature, CO2 concentration in the atmosphere, light, and duration of the growth period. This is why weed grown indoors can have higher concentrations of THC than weed grown outside.
THC dissolves well, as do the other cannabinoids, in fats but not in water. That is why it takes a relatively long time for it to have an effect when cannabis is eaten. By smoking it THC, is rapidly taken into the blood because to get the THC out of the cannabis, it has to be heated. This is usually done by smoking, but can also be done by preparing it in food.
When smoked, between 10 and 25% of the THC is taken up by the body; when eaten, this is barely 6%. The percentage is smaller when eating because a large part of the THC is broken down directly by the liver before it has a chance to reach the brain. The dose of THC needed to make someone feel high is around 10 milligrams, of which only a small portion actually gets into the blood stream and is eventually ferried to the brain.