SAN FRANCISCO – Psychologist Bob Field was puzzled, then anxious, when enrollments plummeted by a third at Quest Therapeutic Camps, his program for children with emotional and social issues.

Eventually he realized the problem was negative reviews on Yelp, which he said were exaggerated or untrue and stemmed from families unhappy over billing. But his hands were tied by medical confidentiality laws from saying so on the online reviews site.

"It really is a bind," Field said. "I wanted to respond [online], but I couldn't. I e-mailed Yelp and said, 'This is libel. How can it be allowed?' and got no response."

His problem is magnified across the medical world, with doctors, dentists, psychologists and other medical professionals saying that Yelp reviews, often used by prospective patients to vet providers, can be wielded as weapons by peevish — or sometimes clinically disturbed — people. The medical professionals say they have few ways to defend themselves.

Ironically, Yelp started when co-founder Jeremy Stoppelman was seeking a doctor to treat his flu and could only find generic information online. Health and medical providers now account for 6 percent of reviews, the sixth-largest category. Shopping (22 percent) and restaurants (18 percent) are the largest.

"Yelp is the bane of many doctors' existence," said Dr. Jonathan Kaplan, a San Francisco plastic surgeon. "A patient can be really vocal, but you cannot. It's not a fair playing field."

At the same time, doctors and others with years of training and experience resent being pigeonholed into the same reviews system used for manicurists or hamburger joints.

"Online opinions of physicians should be taken with a grain of salt, and should certainly not be a patient's sole source of information when looking for a new physician," the American Medical Association said in a statement. "Choosing a physician is more complicated than choosing a good restaurant, and patients owe it to themselves to use the best available resources when making this important decision."

In Field's case, eventually he figured out a strategy. He e-mailed other parents asking them to consider leaving their own reviews. More than a dozen responded, boosting the camp's rating from one star to 4.5 stars. He replied online to the most negative review simply stating that Quest's parent surveys found a 94 percent satisfaction rate. He also tried contacting the negative reviewers privately to address their issues, although that didn't result in any changes.

Field's approach reflects advice from experts and Yelp itself.

"Patients can post very detailed information about themselves and their providers, but the providers have to be very vague when they respond, not even confirming that the reviewer is a patient," said Danika Brinda, who runs Planet HIPAA, a business advising medical professionals on private and security. While providers can post a response, "They cannot even say, 'I'm sorry your visit wasn't good' because that confirms someone was a patient," she said. "Instead they should try to keep it general and vague, focusing on company policy."

Yelp also suggests that medical professionals privately contact reviewers about their concerns. Direct messaging to a business' reviewers is available to any merchant that lays claim to its business on Yelp.

"You don't want to give the wrong impression that you're just calling about a bad review, you want to convey that you want them to let you know directly when they have a problem," Kaplan said.

Darnell Holloway, Yelp director of business outreach, said the site will remove reviews that violate its policies, but those are only the egregious ones: reviews that include hate speech, threats or harassment; reviews that indicate a conflict of interest such as ones posted by competitors; and reviews written by people who do not have direct experience with the provider.

San Francisco psychologist Megan Lehmer has a particularly tricky path. She works as a court-appointed child custody evaluator for divorcing parents who could not work out their own arrangements — often cases involving child abuse, domestic violence or chemical dependency.

"We're very vulnerable to bad Yelp reviews because [clients] are so high-conflict and so angry," she said. "The people who need child custody evaluations are often not nice people and certainly won't stop at using social media for making unfounded allegations."

While Yelp dominates overall user-generated reviews, lots of specialized review sites exist, including several focused specifically on medical providers. These sites generally say that they provide deeper insights, as they ask reviewers to complete checklists on various aspects of care, and often include objective data, on issues like wait times, as well as doctors' hospital affiliations, education and other factors.

But some of the medical-specific sites theoretically could have the same issues as Yelp. While doctors can post responses, they still must follow confidentiality rules. And the sites say they won't remove reviews simply because they are negative, unless there are issues such as threatening, racist or profane language.

Andrea Pearson, executive vice president of consumer products at Healthgrades.com, said people get considerable value out of reading the text of reviews, not just the overall ratings. Perhaps because doctors themselves urge patients to post reviews on the sites, they tend to skew toward more upbeat comments.

"Overwhelmingly it's positive feedback," Pearson said. "People like to like their doctors.''