Among fibromyalgia patients taking either of two commonly prescribed drugs to reduce pain, 22 percent report substantial improvement while 21 percent had to quit the regimen due to unpleasant side effects, according to a new review in The Cochrane Library.

Drugs to treat fibromyalgia just as likely to harm as help, review finds

I would consider a 22% rate of reported improvement an indication of moderately low effectiveness and a 21% rate of reported unpleasant side effects (serious enough to discontinue) to be moderately high risk.