ZURICH, Jan 27 (Reuters) - Roche's cancer drug Tecentriq hit the market months behind immuno-oncology (I/O) medicines from Merck and Bristol-Myers Squibb Co but the Swiss drugmaker's treatment is making up lost ground.

On Thursday, Bristol-Myers managers said Tecentriq is grabbing market share from its I/O drug Opdivo for second-line non-small-cell lung cancer, one factor that forced them to cut their 2017 earnings forecast.

"Most of our erosion in second line has been attributable to" Tecentriq, Murdo Gordon, Bristol-Myers's chief commercial officer, told analysts. "We gave up about 10 points of market share."

Merck's Keytruda drug is also taking patients away from Opdivo, Bristol-Myers said.

The U.S.-based company's muted Opdivo outlook underscores why analysts predict Roche, when it announces full-year 2016 results on Feb. 1, will confirm Tecentriq sales have accelerated, particularly since its initial approval for bladder cancer in May was expanded in October to cover lung cancer.

"Tecentriq continues to show strong launch trends in the fourth quarter 2016," wrote Jeffrey Holford, a Jefferies analyst. "We have been surprised on the upside by its launch."

Life sciences consultancy IMS estimates Tecentriq's May-September sales for bladder cancer hit nearly $70 million, but that does not include lung cancer figures, the biggest cancer market.

Roche declined to provide sales numbers, saying only "we can confirm that we have seen a good launch of Tecentriq".

Tecentriq, Opdivo and Keytruda work by helping the immune system recognise tumors that cloak themselves against detection.

Roche and Bristol-Myers drugs are approved for use after other lung cancer treatments fail, while Keytruda has the FDA's blessing as an initial treatment and is now seeking the go-ahead for combination therapy with chemotherapy, which Merck contends puts it in the pole position in the I/O race.

(Reporting by John Miller; editing by Jason Neely)