Giles Fraser wrote in the Guardian this week about his experiences of low mood, saying that "deviation from social conformity is increasingly seen to be something in need of a pill". Ruby Wax told the Radio 4 Today programme on Monday that GPs, like me, overprescribe medication "like M&Ms … probably to get them off their backs". She went on to say that teachers could recognise a pupil's depression, because in their eyes was a "deadness, not a sadness". This is not a validated test for depression I've ever come across.

We are in a crucial time in medicine, and not just because of the new Diagnostic and Statistical Manual of Mental Disorders or DSM-5, which has provoked numerous media battles between psychologists and psychiatrists, one with a biomedical model of illness and tablets, one with a psychosocial model. It's clear that there has been an enormous overselling of numerous medical interventions – not just in mental health – with overdiagnosis and overtreatment led by an industry keen to get doctors to diagnose as many people as possible. But none of this means that depression – which patients describe to us – doesn't exist.

As with other conditions, such as migraine, there is no blood test or scan to confirm it; diagnosis rests on talking to the person and understanding the symptoms in the context of their life. GPs may take two or three consultations to make the diagnosis and consider medication. Unhappiness is normal; depression, with whatever combination of persistent insomnia, oversleeping, agitation, delusions, anorexia, overeating or suicidal thoughts, is not. Like any sliding scale, there is the potential for definitions to be widened by an industry keen to label more people as depressed. And each person is different in terms of what will help this depression – some people will get on well with therapy, other people will get no benefit from tablets, and vice-versa.

In my experience as a GP, I have learned that many people feel embarrassed and ashamed in telling a doctor about their mental anguish. I've known people who've had both cancer and depression tell me that depression was easily the worst of the two. If people in exquisite mental agony hear a message – no matter how well-intentioned – saying that they may just be merely unhappy, we have failed them. Clearly, doctors can and do get the diagnosis wrong. I prescribe more antidepressants than I would like – but when there's a four- to six-month waiting list for cognitive therapy, how many patients, in distress and suffering, can patiently wait?

When famous people – and there have been many – tell the story of their mental distress, it can be both destigmatising and helpful to others. But there are two problems. One is when assertions of fact are made that go beyond or avoid the evidence. A study in the BMJ in 2009 found that increases in antidepressant prescribing were small, mainly related to a rise in long-term prescriptions. Another study in the British Journal of General Practice on antidepressants in the same year found "little evidence of prescribing without relevant indication". We also know that depression is serious: it has a clear effect on mortality rates and is a risk factor for earlier death.

The second problem is the inference that one's own experience can speak for everyone else's. It can't. The many responses to Fraser's article are united in their difference: who used medication and found a benefit with it, who used therapy, who didn't. Hearing stories of recovery can be extremely useful, but these are individual too.

We must be careful not to stigmatise people who are mentally ill with the idea that they are simply unhappy. Doctors and patients make the best decisions they can with imperfect evidence and a short supply of cognitive therapists. If the wait time for cancer treatment was as long as it is to obtain cognitive therapy, there would rightly be an outcry. So why isn't there?