Side effects were reported to be "infrequent, mild and transient," and included nausea, dizziness and heart and gastrointestinal complaints. The studies reviewed did not examine death rates in detail, however.

Overall, the review showed a "significant benefit in symptom control and physiologic outcomes" in patients who took hawthorn, Pittler said. According to him, the extract appears to boost the strength of heart contractions, increase blood flow through arteries and reduce irregular heartbeats.

There are some caveats, however. Only seven of the 14 trials specified that patients were taking conventional drugs. In addition, patients included in the studies had mild-to-moderate heart failure. Pittler said hawthorn might cause greater side effects in patients who must take drugs that are more powerful to control their disease.

Dr. Gregg Fonarow, director of the Heart Failure Program at the University of California at Los Angeles, said the larger hawthorn study whose results were released last year impressed him more. The study, which was not included in the review because it thus far remains unpublished, found only limited benefit.

"It does indeed demonstrate that [hawthorn] is not a harmful therapy, but it's one that is not particularly helpful nor that would be recommended," Fonarow said. "It's naturally attractive to think there is something over the counter or naturally occurring that may help improve outcome. Unfortunately, we've not been able to identify that so far."