Immediate Effects: The immediate effects of Cannabis will usually vary dependant on the amount used & how it is consumed into the body. Other factors include: The persons previous drug history - particularly with the same drug, their expectations & what mood they are in and finally the way in which the drug is taken (Eaten/Smoked/Etc ). Another major part in the effect of the drug is the quality of it. Obviously the effects would be less severe if the drug was mixed with for example sugar - But more severe if mixed with some type of acidic or poisonous substance or powder. During the first hour after using marijuana the effects are the worst, but the effects can linger around for three hours, sometimes possibly five.
Small Dosage: Small amounts of cannabis will cause the person to have a giddy feeling and have a tendency to talk & laugh more than they usually do. Other effects include reddening eyes, impaired coordination & reduced concentration - It can also affect your ability to drive, much like alcohol.
Larger Dosage: If large doses of cannabis are consumed then a person's perception of time, sound, and colour may become distorted or sharpened. Also the person may feel more anxious, confused and/or excited. As well as all this, the person will still experience the same effect as in small dosages, but to a greater extent.
Source: http://www.drugarm.org.au/main_index/a-z_frame.htm.
Long Term Effects: .
Physical Effects: It is probable that if somebody frequently uses cannabis over a period of years, it helps cause bronchitis & other respiratory disorders with a possible chance of causing cancer in the lunges and other parts of the digestive tact. In addition to this, women who are pregnant and use cannabis risk complications during birth - also the child may temporarily suffer tremor and distress, and can be easily startled.
Mental Effects: Regular users of the drug cannabis can come to rely on it as a social lubricant and feel they need to be under the influence of cannabis almost constantly.