Let's take a quick look at four stocks -- Merck (NYSE:MRK), Idenix Pharmaceuticals (NASDAQ: IDIX), Achillion Pharmaceuticals (NASDAQ:ACHN), and BioDelivery Sciences (NASDAQ:BDSI) -- which could all loom large in health care headlines this Monday morning.

Merck buys Idenix for $3.85 billion
Shares of Idenix are up more than 236% in pre-market trading this morning, after announcing that it had entered a definitive agreement to be acquired by Merck for $24.50 per share in cash. That deal, which has been approved by the boards of both companies, values Idenix at approximately $3.85 billion.

Idenix does not have any marketed products. Its pipeline is comprised of four hepatitis C treatments -- two are in phase 2 trials, another is in phase 1 trials, and another one is still in preclinical trials. Idenix has also been suing Gilead Sciences in the U.S. and Europe, claiming that Gilead's popular hepatitis C drug, Sovaldi, infringes upon its earlier patents.

Back in April, Merck unveiled interim phase 2 results for an oral combination of two experimental drugs (MK-5172 and MK-8742) for the treatment of genotype 1 of hepatitis C, the most common form of the disease. That combination, when administered with ribavirin, achieved a 94% cure rate without the need for interferon, which can cause flu-like symptoms.

Acquiring Idenix is clearly a way for Merck to diversify its hepatitis C pipeline to catch up to Gilead and AbbVie's efforts in the field. Merck shares are down slightly in pre-market trading.

Idenix news boosts shares of Achillion
Shares of Achillion Pharmaceuticals, which also has a pipeline of four experimental hepatitis C treatments, have jumped over 30% in pre-market trading in response to the Idenix acquisition. However, investors should note that no offers have actually been announced for the company.

Investors should also remember that sovaprevir, one of Achillion's most promising pipeline candidates, was put on a clinical hold by the FDA last June due to abnormal liver results in patients. The company had hoped that the hold would be lifted last September, but to no avail. As a result, shares of Achillion have plunged more than 65% over the past 12 months.

The FDA approves BioDelivery Sciences' Bunavail
Shares of BioDelivery Sciences are set to open at a new 52-week high this morning, after the FDA approved Bunavail, its treatment for opioid dependence. The company believes the drug has peak U.S. sales potential of $250 million.

Bunavail is a mucoadhesive buccal film formulation of buprenorphine, designed to compete directly with Suboxone sublingual film. Sales of Suboxone sublingual film came in at $1.3 billion, in the $1.7 billion U.S. market for opioid dependence treatments, according to Symphony Health Solutions.

BioDelivery expects to launch Bunavail in the third quarter of 2014. BioDelivery has reached a deal with Ashfield Market Access to "provide managed markets and trade support" for the treatment. Bunavail will be BioDelivery's second approved product after Onsolis, a treatment for breakthrough cancer pain. However, BioDelivery only receives royalty revenue from Onsolis, since it previously signed the global marketing rights over to Meda.

BioDelivery is also testing another buprenorphine film, BEMA Buprenorphine, in phase 3 trials for moderate to severe chronic pain. Another pain treatment, Clonidine Topical Gel, is also in phase 3 trials for painful diabetic neuropathy.

Shares of BioDelivery have climbed more than 130% over the past 12 months, and are currently up nearly 3% in pre-market trading this morning.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.