The dedicated employees of Google and dedicated editors of Wikipedia do a lot of work to make sure that it is as little of a problem as it is - and there are still persistent issues. If it takes off, the dedicated employees of PatientsLikeMe will have to expect to do the same, and I will expect problems to fall through the cracks the same way they do on Wikipedia and Google.

Strategies for gaming search engines are relatively easy to automatically detect and counteract, once you know what they are. On Wikipedia, a person who knows the subject in question will easily notice fradulent or incorrect information. But I'm not sure what criteria you could use to clearly detect cheaters on a site like this. You could ignore outliers, but that risks losing information from real people who actually have an unusual reaction to the medication. Even if you accepted that as necessary sacrifice, nothing's to prevent the cheaters from creating enough accounts to make them into non-outliers.