The toxicity of cannabis is very low. No one as ever died as a direct and immediate consequence of recreational or medical use of cannabis. But it is wrong to say it is an entirely harmless substance.

smoking
Smoking any drug is unhealthy. Cannabis is no exception. The smoke actually contains higher concentrations of carcinogenic polycyclic aromatic hydrocarbons (PAHs) than tobacco smoke. Cannabis smokers generally inhale more smoke for longer depositing more than 4 times as much tar on their lungs as cigarette smokers. To balance this, however, they smoke less joints and smoke less often.

Worse is if you combine cannabis and tobacco. If you are a heavy smoker of cannabis and tobacco joints (more than 10 a day) you are signicantly increasing your risk of contracting lung disease. Recent studies show that the greatest pre-cancerous abnormalities appear in those who smoke the two drugs together. (1)

Another important factor is that most cannabis smokers stop when they reach their 30s. Long term surveys of cigarette smokers showed that those who stop before the age of 35 had only a very slightly increased risk of lung cancer. The same may apply to cannabis. (2)

paranoia
A common side-effect, usually for first time or early users, is anxiety, panic, paranoia and feelings of impending doom. However these effects disappear within hours. Reassurance and a supportive environment help.

There are also anecdotal reports of long-term users 'turning a corner' in their cannabis use, where they can no longer enjoy the experience as cannabis triggers paranoia and anxiety attacks.

In a recent study, between 10%-15% of people who smoked cannabis reported "paranoid" or "confused" feelings as a disadvantage of smoking cannabis. (2) And over 27% reported "anxiety" as a regular or occasional effect. Around 30% gave "negative experiences" as their reason for permanently quitting cannabis. (3)

schizophrenia
There is a lot of misinformation presented about cannabis and schizophrenia. There is general agreement that heavy cannabis use can precipate schizophrenic episodes in those with the disorder but there is no evidence that it can cause the underlying psychotic disorder. Heavy cannabis users can exhibit long-lasting toxic psychosis involving delusions and hallucinations that can be incorrectly and dangerously - diagnosed as a schizophrenic illness. (4)

If you have mental health problems, taking a drug - any drug - excessively is going to make your problems worse.

1. MARIJUANA and MEDICINE: Assessing The Science Base, pg 111
2. paraphrased from the observations of Les Iversen,
Department of Pharmacology, University of Oxford.
3. Cohen DA, Kaal, Hendrien L, "Irrelevance Of Drug Policy: Patterns and careers of experienced cannabis use in populations of Amsterdam, San Francisco, and Bremen" pg 75
4. ibid, pg 84
5. House Of Lords Select Committee on Science & Technology "Cannabis - The Scientific & Medical Evidence", 4th Nov 1998, pg 12, 4.1-4