Taking a pill to treat depression is widely believed to work by reversing a chemical imbalance.
Medication is a mainstay of mental health therapy
But in this week's Scrubbing Up health column, Dr Joanna Moncrieff, of the department of mental health sciences at University College London, says they actually put people into "drug-induced states".
If you've seen a doctor about emotional problems some time over the past 20 years, you may have been told that you had a chemical imbalance, and that you needed tablets to correct it.
It's not just doctors that think this way, either.
Magazines, newspapers, patients' organisations and internet sites have all publicised the idea that conditions like depression, anxiety, schizophrenia and bipolar disorder can be treated by drugs that help to rectify an underlying brain problem.
People with schizophrenia and other conditions are frequently told that they need to take psychiatric medication for the rest of their lives to stabilise their brain chemicals, just like a diabetic needs to take insulin.
The trouble is there is little justification for this view of psychiatric drugs.
First, although ideas like the serotonin theory of depression have been widely publicised, scientific research has not detected any reliable abnormalities of the serotonin system in people who are depressed.
Second, it is often said the fact that drug treatment "works" proves there's an underlying biological deficiency.
But there is another explanation for how psychiatric drugs affect people with emotional problems.
It is frequently overlooked that drugs used in psychiatry are psychoactive drugs, like alcohol and cannabis.
Psychoactive drugs make people feel different; they put people into an altered mental and physical state.
They affect everyone, regardless of whether they have a mental disorder or not.
Therefore, an alternative way of understanding how psychiatric drugs affect people is to look at the psychoactive effects they produce.
Drugs referred to as antipsychotics, for example, dampen down thoughts and emotions, which may be helpful in someone with psychosis.
Drugs like Valium produce a state of relaxation and a pleasant drowsiness, which may reduce anxiety and agitation.
Drugs labelled as "anti-depressants" come from many different chemical classes and produce a variety of effects.
Prior to the 1950s, the drugs that were used for mental health problems were thought of as psychoactive drugs, which produced mainly sedative effects.
Views about psychiatric drugs changed over the course of the 1950s and 1960s.
They gradually came to be seen as being specific treatments for specific diseases, or "magic bullets", and their psychoactive effects were forgotten.
However, this transformation was not based on any compelling evidence.
In my view it remains more plausible that they "work" by producing drug-induced states which suppress or mask emotional problems.
This doesn't mean psychiatric drugs can't be useful, sometimes.
But, people need to be aware of what they do and the sorts of effects they produce.
At the moment people are being encouraged to believe that taking a pill will make them feel better by reversing some defective brain process.
That sounds good. If your brain is not functioning properly, and a drug can make it work better, then it makes sense to take the pill.
If, on the other hand, we gave people a clearer picture, drug treatment might not always be so appealing.
If you told people that we have no idea what is going on in their brain, but that they could take a drug that would make them feel different and might help to suppress their thoughts and feelings, then many people might choose to avoid taking drugs if they could.
On the other hand, people who are severely disturbed or distressed might welcome these effects, at least for a time.
People need to make up their own minds about whether taking psychoactive drugs is a useful way to manage emotional problems.
To do this responsibly, however, doctors and patients need much more information about the nature of psychiatric drugs and the effects they produce.
Dr Moncrieff's book "The Myth of the Chemical Cure", published by Palgrave Macmillan, will be available in paperback from September.