Jittery. That's probably the best one-word explanation of the market's mood this week. However, investors in a handful of health-care stocks only wish they could be jittery. Instead, they're probably downright jaded after huge share plunges. Here are the week's most horrendous health-care performers.

Holding on
Ariad Pharmaceuticals (NASDAQ: ARIA) emerged as the worst of the worst over the past few days. Shares plummeted 77% this week on news that the U.S. Food and Drug Administration placed a clinical hold on the company's leukemia drug Iclusig.

The key problem is that a significant number of patients taking Iclusig developed serious arterial blood clotting. As a result of the FDA's decision to place a clinical hold on the drug, Ariad has temporarily stopped patient enrollment in clinical studies of Iclusig and reduced the dosage of the drug given to some patients in current clinical trials. The company is also changing the eligibility criteria for Iclusig studies to only accept patients who haven't experienced prior arterial thrombosis resulting in heart attack or stroke.

At this point, there have been no changes to how Iclusig is prescribed for its approved indications of resistant or intolerant chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia. However, the FDA issued a drug safety communication on Friday telling health-care providers that they should "consider for each patient, whether the benefits of Iclusig treatment are likely to exceed the risks of treatment".

Politically damaging
Shares of Acadia Pharmaceuticals (ACAD 0.47%) fell 25% this week, but it wasn't due to the FDA or another company. Instead, Acadia's woes can be attributed primarily to politics -- to be specific, the showdown in Washington over the federal debt ceiling.

For practically all of 2013, Acadia has been on a roll, racking up gains of more than 580%. That roll ended this week when concerns about an impasse on the nation's debt ceiling sent lots of stocks on a downward spiral. Biotech stocks like Acadia were hit especially hard. Some highly visible stock-pickers piled on, saying that it was time to take profits on small biotechs. 

Hopes for Acadia's pimavanserin have driven shares upward. The drug is in a phase 3 clinical study as a treatment for Parkinson's disease psychosis and is in mid-stage studies for schizophrenia and Alzheimer's disease psychosis. Successful results from these studies and subsequent regulatory approval could potentially lead to blockbuster status for pimavanserin down the road.

Jilted
Shareholders of Idenix Pharmaceuticals (NASDAQ: IDIX) are probably thinking, "With friends like Johnson & Johnson (JNJ 0.00%), who needs enemies?" Idenix shares dropped 24% this week after its partner J&J opted to buy rights to a rival's hepatitis-C drug.

J&J and Idenix collaborated together on development of an all-oral hepatitis-C regimen. This week, though, J&J announced that it acquired the rights to GlaxoSmithKline's non-structural 5A protein inhibitor GSK2336805. Although nothing has changed as of yet with the big company's relationship with Idenix, some observers, including Wells Fargo analyst Brian Abrahamas, are now speculating that the partnership between the two companies could fall apart in the days ahead.

2013 hasn't been the best of years for Idenix. In February, the company stopped development of two hepatitis-C drugs. In June, the FDA held up human testing of experimental hepatitis C drug IDX20963 because additional safety data was needed. Now, this apparent jilting by J&J has struck another blow to the stock.

Financial recovery
Which of these horrendous performers appears most likely to recover? Even though the market has seemingly given up on Ariad, I wouldn't completely write off the company yet. Idenix could also see success with its hep-C program and bounce back. 

However, my hunch is that Acadia could be the one to come back most quickly. A deal on the debt ceiling will happen sooner or later. Profit-taking will only continue for a finite period of time. None of the fundamental reasons for buying Acadia have changed. I think this small biotech is one for investors to keep on their watch lists.