Effects of cannabis The effects of any drug (including cannabis) vary from person to person. It depends on many factors, including an individual’s size, weight and health, how the drug is taken, how much is taken, whether the person is used to taking it and whether other drugs are taken. Immediate effects Small doses of cannabis can have effects that last 2–4 hours after smoking. These effects include: relaxation and loss of inhibition increased appetite affected perception of colour, sound and other sensations impaired coordination affected thinking and memory Other common immediate effects include increased heart rate, low blood pressure and reddened eyes.