In theory, this is an unobjectionable standard — what self-respecting doctor could possibly oppose science? — but it also assumes that clinical trials are somehow inviolable, impervious to error. It is Dr. Kramer’s contention that they are not. They are rife with confounds, biases and all-around distortions. Even the shiniest diamonds show cracks beneath his loupe.  “Ordinarily Well” can be slow going. While I delight as much as Dr. Kramer does in scrutinizing studies for flaws in design and execution — he talks effect sizes, dropout biases, additivity — his writing is maddeningly turgid in places. (It’s never a good sign when you’re rereading sentences for the third time.) But stick with him. He has done some much-needed synthesizing and debunking.  Take the meta-analysis, a staple of mental-health research. A meta-analysis compiles the results of many smaller studies to tell us, presumably, what they say in aggregate. All well and good, you might think — until you see how problematic the selection process becomes. Include every study ever done, and you’re bound to include trials that were incompetently designed or only marginally relevant. Include too few, and you might be unconsciously (or deliberately) gerrymandering evidence in your own favor.  “Few discussions of the antidepressant controversy acknowledge this truth,” Dr. Kramer writes. “In psychiatry, we lack large trials, and meta-analyses are imperfect substitutes.”  One of those meta-analyses, not coincidentally, was the first paper aimed directly at Dr. Kramer, published in 1998: “Listening to Prozac but Hearing Placebo: A Meta-Analysis of Antidepressant Medication.” Dr. Kramer argues that the author deliberately “seemed to cull studies in which antidepressants underperformed,” and treated some drugs as placebos even though they may have had antidepressant effects.  Of course, Mr. Kramer also uses meta-analyses to make his own points. Two can play this game. It would be nice if he had acknowledged this explicitly. But his dissections of the most incendiary studies are careful, and his conclusions — that they overestimate placebo effects and underestimate the potency of antidepressants — will invite a reckoning of some kind.  He is particularly devastating on the subject of recruiting test subjects. One of the most damning chapters features an unnamed facility where the Food and Drug Administration routinely conducts at least part of its antidepressant trials. Many of the participants are unemployed or underemployed — lonely, dispossessed and eager for the money. Suddenly, they’re getting paid, interacting with others and receiving the careful attention of doctors and nurses. “Even on placebo,” Dr. Kramer writes, “these patients ought to get better.”  But my favorite chapters, by a long chalk, are his “interludes” describing his own experience treating patients. They are beautiful, philosophical, ambivalent — brimming with all the humility that his opening pages lack.  He tells the story of a patient he calls Olivia. Years after a difficult divorce, she finally starts to date someone — someone who underwhelms her, yet still somehow breaks her heart. “The sequence cries out for psychotherapy,” Dr. Kramer writes. “But Olivia scarcely has the resources to participate in conversation. She recycles recriminations against herself.”  So go ahead. Rate her depression on a scale; assign her a label; try to plot the mess and distress of her life on a graph. But really: What does a double-blind, placebo-controlled trial say about how we can help her specifically, at that moment? “Could a doctor rely on randomized trials only,” he asks, “and treat real people?”  Ultimately, Dr. Kramer seems to be saying, doctors need to treat the human beings who are suffering in front of them. In this case, he puts Olivia on Zoloft to snip the loop of her circular thinking, because he’s seen Zoloft work in these situations many times before. And it works now. She stabilizes. At some point — soon he hopes — he’ll take her off.  Depression is confounding. It resists neat categorization and morphs over time. But the one thing Dr. Kramer finds consistently — in his practice, but also in the research he re-examines in new ways — is that antidepressants work in cases both mild and severe. He cites studies that show antidepressants’ efficacy in other contexts entirely: helping patients in the aftermath of a stroke, for instance, or in countering the effects of the drug interferon.  Dr. Kramer will no doubt set off a new round of debate with this book. But he stands by his story: Antidepressants help people regain custody of their lives. When medicine concerns something so delicate and mysterious as the human mind, it is as much an art as a science. “Doctors don’t see averages; they see patients,” he writes. “In patients, the drugs work.”  310 pages. Farrar, Straus and Giroux. $27.  A version of this review appears in print on June 7, 2016, on page C1 of the New York edition with the headline: The Ardent Defender, Once More to the Antidepressant Ramparts. Today's Paper|Subscribe
Peter D. Kramer, the psychiatrist who wrote “Listening to Prozac,” pushes back against critics who say drug therapy is often no better than placebos.