AbbVie reported slightly better than expected quarterly results on Friday, fueled by surging sales of its Humira treatment for rheumatoid arthritis.

Sales of Humira, which accounts for more than half of AbbVie sales, jumped 16% in the quarter to $2.24 billion and outpaced the company's earlier predictions of low double-digit growth for the medicine for the full year.

AbbVie, the pharmaceuticals business spun off by Abbott Laboratories at the beginning of the year, said it earned $968 million, or 60 cents per share, in the first quarter. That compared with $883 million, or 56 cents per share, in the year-earlier period.

Excluding special items, AbbVie earned 68 cents per share. Analysts, on average, were expecting 67 cents per share, according to Thomson Reuters I/B/E/S.

AbbVie said global quarterly sales rose 3.7% to $4.33 billion, which was in line with the average analyst estimate, according to Thomson Reuters I/B/E/S. Sales would have risen 5.1% if not for the stronger dollar, which hurts the value of sales in overseas markets.

Humira's sales growth in the quarter, typical of its performance in recent years, underscored AbbVie's reliance on the product, which is expected this year to become the world's top-selling prescription drug.

Many investors have fretted that newer treatments for rheumatoid arthritis could slow down growth of AbbVie's injectable drug, including a pill from Pfizer called Xeljanz that was approved in the United States last November.

AbbVie Chief Executive Officer Richard Gonzalez in January shrugged off the Xeljanz threat, telling investors that sales trends for the Pfizer drug were not "worrisome."

Humira's continued growth prospects got a boost late on Thursday when European regulators declined to approve Xeljanz. They cited risk of side effects such as infections, gastrointestinal perforations and malignancies, and questioned how well the pill slowed down structural damage to joints.

Pfizer vowed late Thursday to challenge the decision by the European Medicines Agency's Committee for Medicinal Products for Human Use (CHMP).

Morningstar analyst Damien Conover predicted Xeljanz will eventually be approved in Europe, but not for at least another few years.

"Any delay will meaningfully help Humira over the next few years," Conover said.

European regulators appear to have "raised the bar" for approving drugs for rheumatoid arthritis, Conover said, wanting greater assurance they slow down progression of the disease, and not merely treat pain, swelling and other symptoms.