HUNDREDS of heroin addicts in Hong Kong prisons will be given a methadone substitute every day if the Government adopts a revolutionary plan being examined by experts. The Health Department has asked the Correctional Services Department (CSD) to determine whether it would be beneficial to extend the Government's $20 million methadone programme, now treating about 9,000 people, to prisons territory-wide. But senior CSD officers fear the programme would compromise security and lead to racketeering by prisoners at the Government's expense. ''There is a danger that inmates would demand more than they need and then use it for illegal transactions with other prisoners,'' said CSD Senior Superintendent Pauline Chan Sim-ying, who is conducting an intensive study. Hong Kong prisons accommodate about 2,500 prisoners convicted for drug offences. Senior CSD officers estimate that incarcerated addicts account for up to half the entire prison population of about 11,000, excluding those in Vietnamese detention centres. Dozens of heroin addicted teenagers as young as 14 are serving sentences at CSD drug addiction detention and treatment centres at Hei Ling Chau, Tai Lam and Tai Tam Gap. Drug addicts facing the start of a prison sentence frequently attempt to smuggle narcotics into the CSD's Lai Chi Kok Reception Centre, CSD sources said. Capsules of heroin are swallowed or inserted in body cavities by drug addicts in an effort to maintain their habit while in the reception centre, which is used for detoxification of addicts awaiting transfer to prison. The CSD believes a strict regime of internal searches and urine testing has largely thwarted attempts to traffic drugs in to CSD facilities. But under CSD policy, heroin addicts and those on methadone programmes run by the Health Department cannot receive the harmless substitute once in jail. This has caused acute discomfort as methadone withdrawal can take up to 18 days and heroin withdrawal from six to eight days. An increased risk of suicide or self-harm and a higher chance of drug abuse on release was also common in addicts not being given methadone, according to Health Department sources. On rare occasions the CSD will transfer a detoxifying prisoner to a government hospital for methadone if a doctor says his condition is life-endangering. Methadone programmes in prison systems are not uncommon in Western countries, but it is understood their introduction in Hong Kong would break new ground for Asia. Ms Chan, who is in charge of treatment and programme development for the CSD, said the security implications of a methadone programme in prisons were worrying. Health Department special services director Dr Lam Ping-yan, who is also convinced of the benefits of methadone, shared this anxiety. ''We have to concern ourselves with a number of security aspects and problems that might arise from giving methadone to inmates,'' said Ms Chan. ''I am not sure if it would be ideal for our centres, but it is still under active consideration and we have not yet come to a conclusion.'' The majority of Hong Kong drug offences are related to heroin, which is peddled for as little as $350 a gram. The average addict shoots 0.5 grams of No 4 grade heroin a day. Methadone, usually swallowed in a lime-green liquid form, does not provide the euphoria of heroin, but it occupies the narcotics receptors in an addict and minimises cravings.