(Adds detail, estimates on drug sales; updates shares)

Jan 30 (Reuters) - Pfizer Inc beat analysts' estimate for adjusted profit on Tuesday, helped by strong demand for its pneumonia vaccine Prevnar and rheumatoid arthritis drug Xeljanz, while reporting an $11 billion gain from the new tax law.

The largest U.S. drugmaker also forecast full-year earnings and revenue well ahead of Wall Street estimates and said it would invest about $5 billion in the United States over the next five years.

Pfizer's shares, which traded at a near 16-year high ahead of fourth-quarter results, were down 1.3 percent premarket on Tuesday.

The drugmaker said it would pay about $15 billion in taxes over eight years to bring funds kept overseas back to the United States under the new tax laws.

Pfizer said the 2018 forecast reflected a full-year contribution from the consumer healthcare business, which the drugmaker plans to divest or spin-off this year.

The company said it booked a gain of $11.34 billion from the new tax law, leading to a surge in fourth-quarter profit to $12.27 billion, or $2.02 per share.

Excluding the tax gain and other items, the company earned 62 cents per share. Revenue rose marginally to $13.70 billion.

Analysts on average were expecting a profit of 56 cents per share and revenue of $13.68 billion, according to Thomson Reuters I/B/E/S.

Prevnar raked in sales of $1.53 billion, up 8.3 percent from a year earlier, and above estimates of $1.4 billion, according to consensus estimate from Barclays.

Xeljanz sales jumped 47.5 percent to $410 million, beating estimates of $393 million.

However, sales of the company's breast cancer treatment Ibrance came below consensus estimate, despite an 11.4 percent increase to $716 million.

Pfizer forecast full-year adjusted earnings per share between $2.90 and $3 and revenue of $53.5 billion to $55.5 billion.

Analysts on average were expecting a profit of $2.78 per share and revenue of $53.88 billion. (Reporting by Divya Grover in Bengaluru; Editing by Sriraj Kalluvila)