(Reuters) - U.S. drugmaker Merck & Co beat analysts' estimate for quarterly profit on Friday, driven by higher demand for its blockbuster Keytruda cancer immunotherapy.

Sales of Keytruda, which works by taking the brakes off the immune system, nearly tripled to $1.30 billion in the fourth quarter, compared with consensus estimate of $1.25 billion, according to Barclays.

Merck's net loss attributable widened to $872 million, or 32 cents per share, in the quarter, from $594 million, or 22 cents per share, a year earlier.

The company said it took a provisional charge of $2.6 billion related to the new U.S. tax law. The charge was offset by a 56 percent drop in R&D expenses.

The decline in R&D expenses in the latest reported quarter was driven primarily by lower in-process research and development impairment charges, the company said.

Excluding items, Merck earned 98 cents per share, beating analysts' average estimate of 94 cents, according to Thomson Reuters I/B/E/S.

Revenue rose to $10.43 billion from $10.12 billion.

Merck forecast 2018 adjusted profit in the range of $4.08-$4.23 per share.

Analysts on average were expecting $4.11 per share.

(Reporting by Manas Mishra in Bengaluru; Editing by Sriraj Kalluvila)