Merck (MRK -0.01%) released its first-quarter financial results before the market opened on Tuesday, reporting total sales of $12.1 billion, 11% higher than the year-ago period. Its biggest cash cow -- the cancer drug Keytruda -- continues to sell well. During the first quarter, the company generated $3.3 billion in revenues from Keytruda, a 45% year-over-year increase. Also, Merck's HPV vaccines -- Gardasil and Gardasil 9 -- recorded a 31% year-over-year increase in sales to $1.1 billion.

The gains, however, did not apply across the company's entire portfolio. For instance, sales of diabetes medicine Januvia  fell 6% to $1.3 billion.

Overall, Merck's GAAP net income rose 10% year-over-year to $3.2 billion, and its earnings per share increased 13% to $1.26.

Naturally, the pharma giant mentioned the coronavirus pandemic in its press release: "In response to the COVID-19 pandemic, Merck is focused on protecting the safety of its employees, ensuring that our supply of medicines and vaccines reach our patients, contributing our scientific expertise to the development of antiviral and vaccine approaches, and supporting healthcare providers and our communities," management said.

Doctor standing inside a pharmacy.

Image Source: Getty Images.

The company said that the impact of the pandemic on its Q1 financial results had been immaterial. However, Merck expects the ongoing public health crisis to sap about $2.1 billion from its revenue for 2020. Merck adjusted its guidance in anticipation of those adverse effects. The pharma giant had initially forecast revenue for 2020 to be between $48.8 billion and $50.3 billion, with GAAP EPS of between $4.57 and $4.72. Now, Merck expects its revenue will land between $46.1 billion and $48.1 billion and its GAAP EPS to be between $4.12 to $4.32.

Merck's stock slid by about 3.5% on the heels of its earnings release. The broader market as measured by the S&P 500 index declined by about 0.5% Tuesday.