Merck shares tumbled Friday after reporting a quarterly loss due to the effects of a major cyberattack and costs connected to a drug joint venture with AstraZeneca.

Merck reported a loss of $56 million in the third quarter, compared with $2.2 billion in profits in the year-ago period.

Revenues dipped two percent to $10.3 billion.

The biggest hit came from a $2.35 billion charge connected to the AstraZeneca drug development and commercialization venture, which will expand use of the Lynparza drug, an oral remedy for treating tumors in breast, pancreatic and other cancers.

Merck also said a June cyberattack had led to $135 million in lost sales, plus an additional $175 million in expenses. Chief financial officer Robert Davis said the company expects a similar hit in the fourth quarter.

Merck has said the incident disrupted its manufacturing and formulation operations and that some drug deliveries were delayed.

On the positive side, quarterly sales of Merck's Keytruda anti-cancer drug exceed $1 billion for the first time. 

The treatment, which is being introduced in markets around the world, is most often used in lung cancer, but also regularly employed for melanoma, head, neck and bladder cancer, executives said on a conference call.

Merck also boosted its full-year profit forecast range to $1.78 to $1.84 per share from the prior $1.60 to $1.72 per share due to a lower expected tax rate.

Shares plunged 6.6 percent to $57.82 in afternoon trading, making it the biggest loser in the Dow.