IT is irresponsible to condone the use of marijuana saying it is, ''fun'', harmless and possibly even good for you'' (South China Morning Post, October 16). This substance, we are told, offers a vital role in the treatment of cancer patients, preserves sight of glaucoma sufferers and relieves acute pain amongst other properties.

Here is some additional information. Marijuana, or more accurately THC the active chemical in marijuana responsible for its actions, has been used to help alleviate the nausea associated with chemotherapy in cancer patients. Many had to discontinue as they found the sedation, ''highs'' and even induced nausea were unacceptable. Effective alternatives are available. Marijuana has also been demonstrated to depress immunity. Many cancer therapies aim to increase the body's immune response against the cancer cells. Using such an immune depressant substance is illogical. Glaucoma tends to be a disease of elderly people who cannot tolerate the fast heart rate, drop in blood pressure and sleepiness caused by marijuana. To have any beneficial effect on the eye pressure the chemical needs to be taken on a regular basis.

Marijuana has not been shown to have any pain relieving properties. It may well alleviate some of the anxiety associated with pain, but it is not itself an analgesic. It does however reduce short term memory, cause acute panic attacks, feelings of paranoia and persecution in some individuals. It can cause infertility. Mothers using marijuana during pregnancy have smaller babies than normal. It is associated with an increase in road traffic accidents involving both vehicles and pedestrians. Users are more likely to be taking other mood altering substances.