HEALTH authorities are assembling a task force to stamp out ''under-the-counter'' sales of dangerous and restricted drugs. Government doctors and pharmacists plan a three-pronged attack on the illicit trade, with the launching of the task force, a bilingual telephone hotline similar to the AIDS line launched last year, and a campaign on television, in schools and pharmacies. The hotline strategy urges the public to report pharmacies which illegally sell tranquillisers, antibiotics, psychotropics and other restricted drugs without a prescription. Pharmacists could face maximum fines of $30,000 and a 12-month jail term for selling common antibiotics without prescription. ''Anyone who knows where there are drug stores selling drugs without prescriptions can call and report them,'' said Hong Kong Medical Association spokesman Dr So Kai-ming. Another telephone line will tell callers of the benefits and dangers of various drugs. Patients will be able to hear advice on different illnesses by pressing different digits on their telephones. ''If a person suffers from hypertension, we'll give information on hypertension drugs. If they suffer from heart disease, we'll give advice on heart drugs,'' a Health Department spokesman said. ''It will have information on drug law enforcement in Hong Kong, to make sure people do not shop around dispensaries asking for drugs to be sold illegally to them.'' Family doctors and psychiatrists are concerned at the increasing numbers of teenagers becoming addicted to Valium, Frisium and Mandrax. ''The most notorious soft drug is Mandrax,'' Dr So said. ''It is supposed to be a sedative, but some youngsters are using it to give them kicks, a high. If they overdo it, they could die.'' The medical association has scheduled a press briefing this week to warn of the illegal sales and publicise the hotline. Assistant Director of Health Dr Lam Pin-ying said it was impossible to keep strict control over pharmacists. The fear is that people could go into any number of pharmacies and receive antibiotics, tranquillisers or other drugs without a prescription. Many could be dangerous if used liberally or combined with other pills. ''With 26,000 dispensaries, we cannot go around them every day,'' Dr Lam said. ''The Pharmacy and Poisons Board will write to all dispensers warning them to stay within the law . . . and we'll be sending decoy customers to see how prevalent this is. ''We'll be seeking resources for a special pharmaceutical task force to specifically aim at prosecutions. In a few months' time a whole team of pharmacists will be trained . . . to step up enforcement.'' Government printers are preparing pamphlets to be given out in hospitals and pharmacists, and authorities are deciding on a ''teen idol'' pharmacy educator to appear in schools and on television to promote the campaign.