Reckitt could be fined tens of millions of pounds and be forced to pay up to £40m to the National Health Service.
In a statement, Reckitt – whose stable also includes Dettol, Clearasil and Cillit Bang – said it believed that it competed fairly in all of its operations.
The OFT moved against the company after a whistleblower made the allegation to BBC’s Newsnight in 2008.
The former senior Reckitt executive claimed that the makers of Gaviscon maintained an effective monopoly on the market for years after the stomach medicine came off patent.
They also suggested senior Reckitt Benckiser figures plotted to create obstacles to stop rival manufacturers from selling cheap generic copies.
The whistleblower alleged the firm had “cheated the NHS” and could have saved it "millions of pounds".
Reckitt Benckiser (RB) said in response to the OFT announcement: "The OFT has stressed that no assumptions should be made at this stage. RB believes it competes fairly and within the letter and spirit of the law in all of our operations, and has co-operated fully with the OFT throughout its inquiry.
"Gaviscon Advance is a second generation product, superior to Gaviscon Original. Gaviscon Advance was launched to provide patients with effective and long lasting relief from heartburn and indigestion.
"RB will now review the OFT statement and respond accordingly."