The overall markets experienced the biggest one-day losses in quite a while this week. But those drops were pocket change compared with these horrendous health-care stocks.

Delay-related dive
Shares of small biotech Idenix Pharmaceuticals (NASDAQ: IDIX) took a 32% nosedive this week, following unwelcome news from the U.S. Food and Drug Administration. The FDA told Idenix that it can't start human testing of experimental hepatitis C drug IDX20963 yet because more safety data is needed.

As a result, Idenix was forced to put its planned initiation of the clinical program for IDX20963 on hold. The company will respond to the FDA, but it didn't say how long that effort would take.

Idenix has one marketed product thus far -- hep-B drug Tyzeka. However, the company sold the rights to the drug to Novartis (NVS 0.84%) in 2003. At that time, Novartis also bought a 54% stake in the small biotech. The two companies restructured their collaboration agreement last year. As a result, Idenix no longer receives any royalties or milestone payments for Tyzeka. Over the years, Novartis' stake in Idenix has decreased considerably, although the large pharmaceutical is still a major shareholder.

Dominoes deliver
Genmark Diagnostics (GNMK) seems to be playing a not-so-fun game of domino-stacking. Those dominoes delivered an 18% fall in the company's share price this week.

The company made our horrendous-stocks list last week because its biggest customer, Natural Molecular Testing Corporation, or NMTC, struck a deal to use technology from one of Genmark's chief rivals. This week, Genmark announced the next domino to fall as a result of that deal. The company lowered its full-year revenue guidance by 14%, citing uncertainties relating to NMTC's decision and reimbursement challenges.

On the positive side, Genmark still sees 2013 revenue growing to 50% higher than last year's revenue. CEO Hany Massarany noted also that the company is making good progress in the development of its new NexGen molecular diagnostic testing system. Massarany said that Genmark believes that the sample-to-answer system will "fuel significant growth" for the company in the future.

Downgrade downer
An analyst downgrade served as a downer for Tesaro (NASDAQ: TSRO) stock this week. Shares dropped 17% after BMO Capital Markets changed its rating on Tesaro from "outpeform" to "market perform."

Tesaro had great news earlier this month. The company's phase 1 results for niraparib in treating ovarian cancer were very positive. Niraparib also had good results in a study focusing on the treatment of breast cancer and prostate cancer. 

However, BMO Capital Markets analyst Jim Birchenough thinks that "the field is getting crowded" already for niraparib, with several drugs on the scene that use similar mechanisms. Birchenough said the niraparib results, as well as positive data for chemotherapy-induced nasea and vomiting drug rolapitant, are reflected in the valuation of Tesaro shares. Although the stock was downgraded, BMO maintains a $36 price target for Tesaro.

Spending some pocket change
Which of this week's three horrendous stocks might still be worthy of throwing some pocket change its way? Any of the three could rebound, but doing so won't necessarily be easy. My view is to hold off on buying any of these stocks at this point.

There's too much uncertainty with the FDA's concerns to jump into Idenix. I'd wait to see how well Genmark recovers from the big hit from NMTC's swinging business to its rival. Tesaro could be the best pick, but the stock could experience considerable volatility. Sometimes the best place for pocket change is in your pocket.