Dark circles under the eyes, lack of drive, sleepy look: None of the things that expose the stoner as a cliché can be seen in Renate Büchi's son. The grammar school pupil is sporty, open-minded and a good student. It's only the kitchen grater that blows his cover. His mother wants to grate fruit, but the appliance has disappeared. She finds it a few days later in her son's room. The grater is sticky and discoloured brown. Renate Büchi thinks nothing of it, cleans the appliance and puts it in the kitchen. When she goes to grate cheese, the grater is missing again. Her mum doesn't have to look far. "Do you eat that much fruit?" she asks her son at dinner. The four siblings cast furtive glances at each other and grin. Weeks later, Büchi reads about cannabis in a magazine. It says that hashish is crushed before use - with a kitchen grater, for example. That's when it clicks.

How could we not notice? "The question plagued me," says Büchi. Today, a good decade later, she encounters many parents who feel the same way she did back then. For a few years now, the psychiatrist has been part of the team of experts at the Samowar youth counselling and addiction prevention centre in Horgen, Zurich, advising young people, parents and schools. There is no conclusive answer to the question of how drug use manifests itself, she says. In the case of weed, it could be the intrusive sweet smell of weed that parents notice, apathy or social withdrawal that emanates from the young person. However, Büchi knows that it is not uncommon for parents to notice nothing at all.

So the child is smoking pot. The anger is followed by concern: smoking weed is harmful. You want to make your son or daughter realise the seriousness of the situation. But how? "Parents often don't manage to get through to their child," says Felix Hanselmann from the Zurich Oberland Addiction Prevention Centre. "Out of fear, they start making threats and unrealistic demands: 'If you don't stop smoking weed, you won't go out again! Parents know for themselves that it's difficult for them to follow through." Although this behaviour is understandable, it sends a bad signal to the young person because they realise that their parents are just as overwhelmed as they are.

Prevention expert Büchi's advice is not to act on instinct. This means letting anger and annoyance subside a little, planning a conversation - and preparing for it. Parents should think in advance about which points they want to address, what they want to find out, but also be clear about which accusations they don't want to throw at the child under any circumstances. "This increases the chances of the discussion going reasonably well," says Büchi. A good time to sit down together is in the evening, when there are no more appointments or homework to do.

"I've noticed that you smoke pot, that worries me." I-messages are recommended because they don't push the other person into a corner. If you are hoping for openness from your child, you should at least try not to put them on the defensive. "You've been smoking weed behind our backs!" This, says Büchi, is probably what she would have done if she had confronted her son as soon as she found out about him. However, she knows that accusations make adolescents even more stubborn. "It's worth preparing your message so that it doesn't leave the child unaffected." At the same time, parents shouldn't expect too much, says the expert: "If the son or daughter provides information, that's enough for now. The child will probably not respond to the parents' concerns."

Büchi realised at the time that her own lack of knowledge about cannabis was a cause for concern. "Today, I would recommend a mother in the same situation to turn to a counselling centre with her questions," she says, "and not just when there's a fire." The "Elterninfo Cannabis" hotline run by Addiction Switzerland, for example, provides help during office hours on the freephone number 0800 104 104. The cantonal youth counselling, addiction prevention and specialist drug centres are also there for parents. Their services are usually free of charge and include information and counselling. Education is also worthwhile if smoking pot at home is not an issue. The addiction prevention centre of the city of Zurich, for example, organises the parents' evening "Talking about cannabis" on request, and the Bernese Health Foundation makes its experts available to parents on the same principle.

"Parents expect changes too quickly," says health sociologist Hanselmann. "But habits don't change overnight." He advises parents not to panic. Smoking pot is a youth phenomenon, but it doesn't automatically have to be an addiction problem. If parents try to persuade young people to make agreements, this is more effective than scolding and threatening: "For example, you can agree to take breaks from smoking because it's important for their health." Expressing your own concerns and at the same time signalling a willingness to compromise can help to maintain a connection with the child. If things get out of hand, an outside perspective can help: "A godfather, a family friend - they can break down hardened fronts."

But staying in dialogue doesn't mean constantly talking about weed. "No matter how difficult a phase of life is," says Renate Büchi, "it always has beautiful things to offer." It's worth focussing on this too - especially when dealing with your son or daughter. "There are certainly other things to talk about: Music, sport, topics that connect us."

Not rushing into things and yet remaining vigilant is often a balancing act for parents. For example, the question of when professional help is needed. "We shouldn't lump all stoners together," warns Pierre-André Michaud, former head of the multidisciplinary department for adolescent health in Lausanne. In his view, however, there is a need for treatment or at least counselling if cannabis use leads to complications such as delinquency, exclusion from school or accidents, if young people use cannabis because they want to solve personal problems or if schools and employers push for treatment. Parents cannot force their child to undergo counselling - but they can motivate them with the prospect that they will be allowed to talk to specialists on their own.

Hemp plants with a THC content of over 1 per cent are considered drug hemp and their consumption, trade and cultivation are punishable by law. Since October 2013, cannabis consumption in Switzerland has been punishable by a fine of CHF 100, provided that no more than 10 grams of the substance are involved. However, this does not apply to minors. If they are caught smoking weed, the police usually inform their parents and the youth welfare office. The practice varies from canton to canton; the younger the users are, the more action is taken. Young people over 15 who are caught for the first time are usually given a warning. Younger offenders and those who are caught again are investigated by the youth welfare office.

Possible penalties include a reprimand, community service or a fine. "We don't want to criminalise young people," says Patrik Killer from the Zurich Youth Ombudsman's Office, "but rather try to understand their situation as a whole." If, in the course of the investigations, it turns out that cannabis use is significantly jeopardising the young person's further development, the youth welfare office can also order protective measures such as therapeutic treatment. However, according to Killer, proportionality is always paramount. For example, compulsory attendance at a course that addresses the risks of cannabis is one of the most common sanctions in German-speaking cantons.

Whether the whole thing will end at some point is the most pressing question being asked by concerned parents. "We know from long-term studies that cannabis use is highest among 15- to 24-year-olds, that it is already declining among 25- to 34-year-olds and that it decreases sharply with every year of age," says Frank Zobel, Deputy Director ad interim at Addiction Switzerland. "Most young people who try cannabis only smoke it occasionally and quickly stop using it again. Of the minority who use regularly, most stop between the ages of 20 and 30 - often when they start working or start a family." The son of prevention expert Büchi belongs to this group: he stopped smoking weed when life became more strenuous due to his studies and job.