Cannabis induces a state of intoxicated relaxation, euphoria and dreaminess.

It is quick acting. Smoked, the first effects are usually felt within seconds. The peak follows within minutes and then declines sharply after about forty five minutes, with a ninety minute tail-off.(1)

You feel a general sense of well being and relaxation, giggliness and euphoria. You may also experience introspective dreaminess, increased appreciation of music, sleepiness and time distortion. The effects can be subtle. First time users often detect little or no effect.

Physically, bloodshot eyes and a dry mouth are common symptoms, as well as a slight increase in heartbeat and impaired short term memory.

Cannabis increases heart rate but decreases blood pressure, like a car changing down a gear (2) Regular users gain tolerance to this effect.

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in the brain
Cannabis has two powerful active ingredient - THC and CBD (cannabidiol). Both substances are classed as cannabinoids - they produce psychoactive effects by binding with special receptors which are extraordinarily abundant all over the brain and body. Your brain makes its own cannabinoid - anandamide - which is thought to be involved in pain sensations, memory regulation and the immune system. (3)

The ratio of these two substances determine the character and strength of the effect. Like fine wines, different strains of cannabis can have their own flavour and complexity, plus their own distinct high.

a note on eating cannabis
If you take it orally, you could be in for quite a ride. The liver metabolises THC into 11-OH-*9-THC, a compound three times more psychoactive. Oral cannabis is a powerful, almost psychedelic experience, very uncomfortable if you're not expecting it. The effects are slower to come on (1 to 2 hours) and last considerably longer (around 4 hours)

after effects
Cannabis can leave you feeling a little groggy and forgetful but little else. Depending on how much you've smoked and its character and strength, these effects can linger for hours or days.

1. Harder S, Rietbrock S (1997) Concentration-effect relationship of delta-9-tetrahydrocannabiol and prediction of psychotropic effects after smoking marijuana. Int J Clin Pharmacol Ther 35(4):155-9
2. UK Independent Drug Monitoring Unit, “Cardiovascular Effects Of Cannabis”
3. MARIJUANA and MEDICINE: Assessing The Science Base, pg 43