Biotech's the biggest boom-or-bust business on the market, with regulatory approvals and clinical trial results routinely sending stocks hurtling up or down by significant amounts. The industry had a tough week as a whole, with the Nasdaq Biotechnology Index losing a gut-churning 2.7% over the past five days. But in an industry like this, there are always big gainers and big losers every week -- and this was no exception.

From second-quarter earnings success to a slumping outlook at one closely watched company, these stocks rewarded and plagued investors this week to the tune of double-digit gains and losses.

Dendreon takes a plunge
If you're looking for one biotech stock more responsible than any other for the NBI's fall this week, look no further than Dendreon (NASDAQ: DNDN). Dendreon's shares fell more than 26% this week, after the company announced that it expects little to no growth from cancer drug Provenge in 2014. Sales of the drug already are on the downswing, as Provenge's revenue declined by more than 8% for the second quarter -- part of a 13% drop in the drug's sales over 2013's first half.

Analysts were once high on Provenge's potential, with some expecting the drug to become a blockbuster product with billions of dollars of revenue. That hasn't panned out for Dendreon and its investors, particularly as all-oral prostate cancer therapies, such as Johnson & Johnson's (JNJ -0.69%) Zytiga, have made Provenge's injectable therapy look inconvenient by comparison. Given that Zytiga's posted strong double-digit sales growth for the year's first half and is on pace to eclipse blockbuster status by the year's end, it's clear just which medications are getting ahead in this area.

Dendreon might still have appeal through getting its drug approved in Europe, but this stock's potential is fading fast.

It wasn't such a bad week for shareholders of Idenix Pharmaceuticals (NASDAQ: IDIX), however. Shares of the biotech grew 12% for the week after Idenix reported a smaller earnings-per-share net loss in the second quarter as compared with last year's Q2. The company lost $0.22 per share in earnings in the most recent quarter, compared with $0.23 a year ago.

It's hardly market-shaking news, but Idenix's real battle is ahead of it. The company's angling to become one of the early entrants into the all-oral hepatitis C market, a market that some analysts have predicted could exceed $20 billion in annual sales in the near future. It's a lucrative opportunity for investors, but Idenix has fallen behind the early leaders in the race, AbbVie and Gilead Sciences (GILD 0.07%). Gilead's sofosbuvir has looked strong in early-stage clinical trials, and analysts have pegged peak sales estimates of the drug at up to nearly $4 billion. The potential's still there for this company to make some real noise in this growing market, but Idenix will have to act fast to catch up with its much larger rivals.

The week's big winner, however, was none other than NPS Pharmaceuticals (NASDAQ: NPSP). I've been high on this growing biotech firm before, but shares blasted off this week by 21%, part of a whopping 149% gain year-to-date.

NPS boosted its revenue by double-digit percentage growth in the most recent quarter, but all eyes are on Gattex, the company's recently approved short bowel syndrome, or SBS, therapy. SBS is a rare disease, so prescriptions of the drug are only slightly above a hundred -- but for an expensive drug like this, that's been enough to push early sales of Gattex to $4.8 million for the most recent quarter. It's strong progress for NPS, especially as the company expects to submit its next orphan drug, Natpara, for regulatory approval later in the year.

As of now, it's all systems go at this flourishing young biotech. For investors who got in early with NPS, the rewards have been astronomical.