I am quoting from a review of three books by Marcia Angell on this issue that I have personal views on from experience:
None of the three authors subscribes to the popular theory that mental illness is caused by a chemical imbalance in the brain. As Whitaker tells the story, that theory had its genesis shortly after psychoactive drugs were introduced in the 1950s. The first was Thorazine (chlorpromazine), which was launched in 1954 as a “major tranquilizer” and quickly found widespread use in mental hospitals to calm psychotic patients, mainly those with schizophrenia. Thorazine was followed the next year by Miltown (meprobamate), sold as a “minor tranquilizer” to treat anxiety in outpatients. And in 1957, Marsilid (iproniazid) came on the market as a “psychic energizer” to treat depression.
In the space of three short years, then, drugs had become available to treat what at that time were regarded as the three major categories of mental illness—psychosis, anxiety, and depression—and the face of psychiatry was totally transformed. These drugs, however, had not initially been developed to treat mental illness. They had been derived from drugs meant to treat infections, and were found only serendipitously to alter the mental state. At first, no one had any idea how they worked. They simply blunted disturbing mental symptoms. But over the next decade, researchers found that these drugs, and the newer psychoactive drugs that quickly followed, affected the levels of certain chemicals in the brain.
When it was found that psychoactive drugs affect neurotransmitter levels in the brain, as evidenced mainly by the levels of their breakdown products in the spinal fluid, the theory arose that the cause of mental illness is an abnormality in the brain’s concentration of these chemicals that is specifically countered by the appropriate drug. For example, because Thorazine was found to lower dopamine levels in the brain, it was postulated that psychoses like schizophrenia are caused by too much dopamine. Or later, because certain antidepressants increase levels of the neurotransmitter serotonin in the brain, it was postulated that depression is caused by too little serotonin. (These antidepressants, like Prozac or Celexa, are called selective serotonin reuptake inhibitors (SSRIs) because they prevent the reabsorption of serotonin by the neurons that release it, so that more remains in the synapses to activate other neurons.) Thus, instead of developing a drug to treat an abnormality, an abnormality was postulated to fit a drug.
That was a great leap in logic, as all three authors point out. It was entirely possible that drugs that affected neurotransmitter levels could relieve symptoms even if neurotransmitters had nothing to do with the illness in the first place (and even possible that they relieved symptoms through some other mode of action entirely). As Carlat puts it, “By this same logic one could argue that the cause of all pain conditions is a deficiency of opiates, since narcotic pain medications activate opiate receptors in the brain.” Or similarly, one could argue that fevers are caused by too little aspirin.
But the main problem with the theory is that after decades of trying to prove it, researchers have still come up empty-handed. All three authors document the failure of scientists to find good evidence in its favor. Neurotransmitter function seems to be normal in people with mental illness before treatment. In Whitaker’s words:
Prior to treatment, patients diagnosed with schizophrenia, depression, and other psychiatric disorders do not suffer from any known “chemical imbalance.” However, once a person is put on a psychiatric medication, which, in one manner or another, throws a wrench into the usual mechanics of a neuronal pathway, his or her brain begins to function…abnormally.
Carlat refers to the chemical imbalance theory as a “myth” (which he calls “convenient” because it destigmatizes mental illness), and Kirsch, whose book focuses on depression, sums up this way: “It now seems beyond question that the traditional account of depression as a chemical imbalance in the brain is simply wrong.”
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