Fifteen-year-old Sarah is an accomplished violinist. But lately she seems to be losing interest in the instrument - and in any kind of extracurricular activity. Her grades have dropped and she's also having problems focusing at home.
Meanwhile, Andrew's parents are concerned that he is suffering from depression. The 17-year-old seems more aloof from his family, often hanging out with his friends at a nearby park on weekends. He suffers frequent colds and bouts of bronchitis, and sometimes his eyes appear very red.
What do the teens have in common? Both are using drugs.
Young people aged under 21 years form the biggest proportion of new drug users (one third of users or about 1,000 teens) according to the 2012 Central Registry of Drug Abuse (CRDA) survey , which compiles statistics reported by local hospitals, clinics and other agencies.
Although the rates of teen drug use in Hong Kong are lower than in Western countries, there is a significant concern that reported cases are just the tip of the iceberg, as a lot recreational drug use among local teenagers remains hidden.
Adolescence is an important time for physical and psychological development. Using addictive substances at this juncture is particularly dangerous because young people's brains are still growing. However, there is a widespread misconception among teens that certain drugs such as ketamine and marijuana are less harmful than "hard" narcotics such as heroin and cocaine.
Research shows that the earlier teens use drugs, the earlier they become addicted. Ketamine, methamphetamine (Ice), cocaine and cannabis (marijuana) are most popular with local young people under 21, followed to a lesser extent by heroin, cough medicine and Ecstasy.
Ketamine, a quick-acting anaesthetic, is usually snorted as a powder and causes users to hallucinate and lose their sense of time and reality. Teenagers such as Sarah like ketamine because it is cheap, easy to use, and can get them high quickly. However, frequent ketamine use is associated with problems related to long-term memory and co-ordination.
Andrew uses marijuana, which is the most popular drug among teens in countries such as the US and Britain because of its relaxing qualities. Teens can also become dependent on it to feel good and handle stress. It is widely regarded as a gateway drug, as frequent use can lead to the use of stronger substances.
Ecstasy, although less common in Hong Kong, is known as a club drug because it is widely available at raves, dance clubs and concerts. This drug combines empathogenic and stimulant effects, making emotions more intense. Ecstasy can also cause cramps, nausea, seizures and brain damage.
Several over-the-counter cough and cold medicines contain the ingredient dextromethorphan. Known as DXM or DM for short, it is often extracted from cough medicines, converted into powder, and snorted. If taken in large quantities, DXM can cause hallucinations, "out-of-body" experiences, even brain damage and death. In the US, some forms of cough medicine are no longer available over the counter because of the potential for abuse.
Perhaps because of Hong Kong's position as an international hub, youth drug use overseas influences trends here. Prescription drug use is one example. Up to 20 per cent of American teenagers say they take drugs such as antidepressants or medications for attention deficit hyperactivity disorder without a proper prescription. Younger teens turn to inhalants - including household products such as glues, paint thinners, petrol and hair spray - or substances that are sniffed or "huffed", to get an immediate rush. Users often become giddy and confused and can become dependent on inhalants to feel good or handle stress, in addition to suffering severe headaches and heart arrhythmia.
A CRDA survey found that the most common reasons for teenage drug use in Hong Kong were to relieve boredom, depression or stress, to get high, and because of curiosity and peer pressure. Other factors included high pressure to perform well academically, family neglect and poor parental communication, loss of traditional values and an increasingly materialistic society that stresses instant gratification.
A number of schools have instituted compulsory random urine or hair sampling. Students who test positive for drugs may face further tests, compulsory counselling and, in some cases, suspension or expulsion.
But the jury is still out on the usefulness of routine testing. Supporters argue that it helps enforcement and can act as a deterrent to students, but opponents feel that it is unlikely to change behaviour and is largely punitive.
Individual screening and counselling by health care providers can help identify youngsters with potential problems; unfortunately, teens are rarely asked about alcohol or drug use during routine health check-ups. A study conducted at Chinese University found that only 1.3 per cent of university students were probed about alcohol or drug use by their health care provider over the past five years. Standardised written questionnaires issued during routine office visits could serve as a good way to broach sensitive questions such as drug use and other high-risk behaviour.
What can parents do? Many of the initial signs of drug or alcohol abuse may appear to be normal teen behaviour. Mood swings or behavioural changes are a part of growing up. However, parents' instincts are often right. They may stand at different ends of the parenting spectrum - from aggressive and protective to permissive - but they must learn to provide love and support to their teens while maintaining clear boundaries.
Some parents try to hide their child's drug use. A long-standing perception that addiction reflects poorly on the family keeps many teens from getting treatment until the problem is too severe to ignore.
If a teenager is using drugs, start by consulting a family doctor, paediatrician or school counsellor about specialists or treatment centres that can help. Go to drugfree.org/  or kidshealth.org/kid/  for more information. firstname.lastname@example.org 
Dr Anisha Abraham is a paediatrician specialising in teen health and an honorary associate professor at the Chinese University