This is a really interesting website (www.thennt.com)! It basically reviews medical trials and presents the results in a way someone without a degree in statistics can understand. The "NNT" refers to the number of patients doctors need to treat to see a particular result.
In regards to taking antibiotics for sinusitis it found (after reviewing five trials including approximately 500 participants) that:
1 in 15 were helped (faster resolution of symptoms)
1 in 8 were harmed (medication side effects)
1 in 71 did not tolerate the antibiotics
A staggering 93.2% of participants saw no benefit. Check out the link if you're interested! http://www.thennt.com/antibiotics-for-sinusitis/