Elderberry may help treat cold and flu symptoms by reducing congestion and possibly making you sweat more. One study suggested that using a standardized elderberry extract, Sambucol, could shorten the duration of flu by about 3 days. Sambucol contains other herbs plus vitamin C, so no one knows whether elderberry by itself would have the same effect.
Another preliminary study found that a lozenge with elderberry extract (ViraBLOC) helped reduce flu symptoms when taken within 24 hours of symptoms starting. In the lab, one study suggested that elderberry could kill the H1N1 virus ("swine flu") in test tubes, but researchers do not know whether it would be effective against H1N1 in people.