Delasau produced a sample which tested positive for a metabolite of cannabis, a prohibited substance on the World Anti-Doping Code list, following Fiji's 91-0 defeat to New Zealand in June.
FRU chairman Keni Dakuidreketi told TVNZ: "We hope this sends a clear message to all rugby players in Fiji that we will not tolerate drug users in our sport.
"This incident has brought the game into disrepute, and it happened when Delasau was wearing the white jersey and representing his country."
Delasau has made 21 Test appearances for Fiji and is an integral member of their successful sevens side.
The suspension applies to matches under the FRU's jurisdiction, which means he is free to continue his NPC commitments with Canterbury and the 2006 Super 14 campaign with the Crusaders.