The ban was imposed by Australia's National Rugby League in November 2016.
Barba, 27, recently left French rugby union club Toulon, who he joined in a controversial code switch in February.
The Rugby Football League says the NRL's sanction will apply, but clarity is needed to determine if the ban is applicable outside that competition.
Barba tested positive for cocaine and was suspended shortly after helping Cronulla Sharks win the NRL title.
"The initial stance of the RFL is that this also applies to Super League," said a club statement announcing his arrival on a deal until 2019.
"However, St Helens will be seeking further and final clarification of this position."
Should the suspension stand and begin immediately, Barba will not be eligible to feature for Saints until the fourth match of the Super 8s, which is likely to be in August.
Barba, who was the NRL's player of the year in 2012, has scored 99 tries in 168 NRL matches for Canterbury Bulldogs, Brisbane Broncos and Cronulla.
Saints head coach Justin Holbrook said: "I am confident that he will add a new dimension to Saints and is a great capture for Super League. He is a multi-talented and world-class rugby league player.
"It is not often that players of his calibre become available and the club has pulled out all the stops to fight off competition from both rugby codes around the world."
Barba added: "I feel very privileged to be joining such a historic rugby league club.
"I wanted to make an early decision so that I can get my family settled in England and kids enrolled at school. I have certainly enjoyed my time in France at Toulon, but I am a rugby league player and St Helens, especially with Justin as head coach, have made me very comfortable with the decision."
St Helens have signed full-back Ben Barba, but are "seeking further and final clarification" about whether he must serve a 12-match drugs suspension.