I'm going to give some details, and link to resources if you wish to research further.
Long-term use of marijuana produces changes in the brain that are similar to those seen after long term use of other major drugs of abuse such as cocaine, heroin, and alcohol. Moreover, these changes may increase a user's vulnerability to addiction to other abusable drugs by "priming" the brain to be more easily changed by drugs in the future. This study is published in today's issue of Science.
http://www.pslgroup.com/dg/2D4A6.htm
What are the short term effects of Marijuana?
Lack of short-term memory
Illogical thinking
Excitement Anxiety or panic
Impaired coordination impaired motor ability
Increased appetite
Altered perception
Sleepiness
Cannot focus attention
Increased heart rate
Bloodshot eyes
Dry mouth and throat
Paranoia
Hallucinations
Confusion
Restlessness
What are the long term effects of Marijuana?
Slow, confused thinking
Memory impairment
Cancer
Chronic bronchitis
Lack of motivation
Breathing problems
Marijuana also burns at a much higher temperature then tobacco, so it does much more damage to your lungs and does it a lot faster.
Marijuana can cause problems in the reproductive system. It can decrease testosterone levels for men, lower sperm counts, and create difficulty in having children.
Marijuana can Increase testosterone levels for women and lead to increased risk of infertility.
Marijuana diminishes or extinguishes sexual pleasure.
Marijuana increases psychological dependence forcing the user to require more of the drug to get the same effect.
http://www.ayn.ca/health/en/addictions/addiction_marijuana.asp
* Interesting article about medicinal marijuana use
This is how marijuana works: we have receptors for delta-9-tetrahydrocannabinol (THC) – the chief marijuana component responsible for the "high" – in the parts of the brain responsible for memory, mood, motor activity and concentration. Smoking marijuana affects those functions, but because we don’t have receptors in the parts of the brain that control basic life functions (consciousness and respiration), it can’t kill us. For those with MS and epilepsy, marijuana is reported to reduce muscle spasms and to relieve accompanying bladder and bowel incontinence. Many people who suffer from glaucoma – a condition usually resulting from excess pressure in the eye that, if left untreated, can lead to blindness – also stand by marijuana for its effectiveness in reducing the pressure behind the eye. And the mood-improving qualities of marijuana are said to be useful treatment for a range of chronically ill patients.
http://www.thecompassionclub.org/club/chatelaine.html
* Not as bad as smoking? I think not
Effects on of the THC content, the amount of tar inhaled by marijuana smokers
Someone who smokes marijuana regularly may have many of the same respiratory problems that tobacco smokers have. These individuals may have daily cough and phlegm, symptoms of chronic bronchitis, and more frequent chest colds. Continuing to smoke marijuana can lead to abnormal functioning of lung tissue injured or destroyed by marijuana smoke.
Regardless, carbon monoxide absorbed are three to five times greater than among tobacco smokers. This may be due to the marijuana users inhaling more deeply and holding the smoke in the lungs
* Not addicting?
A drug is addicting if it causes compulsive, often uncontrollable drug craving, seeking, and use, even in the face of negative health and social consequences. Marijuana meets this criterion. More than 120,000 people seek treatment per year for their primary marijuana addiction. In addition, animal studies suggest marijuana causes physical dependence, and some people report withdrawal symptoms.
http://www.narconon.ca/English/Marijuana.htm
Quite interesting article written by a psyc prof at SFU
http://www.sfu.ca/~wwwpsyb/issues/1996/spring/ramachandran.htm
* Does it cause cancer?
A breakthrough report published in the journal Science, October 18, 1996,provides the first true molecular evidence conclusively linking components in tobacco smoking to lung cancer.
A chemical found in tobacco smoking, benzopyrene, causes genetic damage in lung cells that is identical to the damage observed in the DNA of most malignant tumors of the lungs. Although scientists have been convinced in the past that smoking causes lung cancer, the strong statistical associations did not provide absolute proof. This paper absolutely pinpoints that mutations in lung cancer cells are caused by benzopyrene.
An average marijuana cigarette contains 30 nanograms of this carcinogen, compared to 21 nanograms in an average tobacco cigarette (Marijuana and Health, National Academy of Sciences, Institute of Medicine report,1982). This potent carcinogen suppresses a gene that controls growth of cells. When this gene is damaged, the body becomes more susceptible to cancer. This gene, P53, is related to half of all human cancers and as many as 70% of lung cancers.
Clearly marijuana smoke contains more of the potent carcinogen benzopyrene than tobacco smoke. Furthermore, the technique of\line smoking marijuana by inhaling deeply and holding the smoke within the lungs presents a chance of much greater exposure than a conventional tobacco cigarette.
http://www.sarnia.com/groups/antidrug/rltychck/cncrlink.html