Oseltamivir (Tamiflu) is commonly used for influenza because it has been shown to reduce symptom duration (by about one day) and was believed to also reduce complications of influenza. The strongest evidence for reducing complications was a review of 10 trials (Arch Intern Med. 2003;163:1667-1672). This meta-analysis was the primary supporting data for this conclusion in a Cochrane review (Cochrane Database Syst Rev. 2006;3:CD001265).
On updating this Cochrane review, the authors investigated this meta-analysis further and found most of the supporting data to be from unpublished trials. They were unable to obtain the original data despite multiple attempts, so these were from their analysis. No statistically significant benefit for oseltamivir for reducing complications was found based on a meta-analysis of three trials.
The updated review still concludes that neuraminidase inhibitors (oseltamivir or zanamivir) reduce duration of influenza symptoms when started within 48 hours and have efficacy for influenza prophylaxis (BMJ. 2009;339:b5106).