The battle for prostate cancer treatment market share intensified in 2013 as Johnson & Johnson's (JNJ -1.15%) Zytiga and Medivation's (MDVN) Xtandi captured business from Dendreon's (NASDAQ: DNDN) Provenge.

It's an important market for these competitors, because prostate cancer is the second-mostwidely diagnosed cancer among men, with an estimated 192,000 new cases diagnosed annually in the United States. That means these three drugs are battling for share in a market worth $12 billion a year in the United States.

With that market expected to grow to as much as $19 billion by 2020, here's what you need to know about each of these treatments heading into next year.

Leading the market

Following FDA approval to expand its label to include pre-chemotherapy prostate cancer patients, Johnson's Zytiga posted impressive sales growth in 2013. A bigger patient pool, combined with a roll-out overseas, resulted in third quarter sales of $198 million -- double last year.

Johnson's Zytiga initially gained FDA approval as a treatment for metastatic castration-resistant prostate cancer, or mCRPC, in April 2011. That approval was based on a phase 3 trial showing that Zytiga patients' overall survival improved by 4.6 months compared to placebo. Importantly, in that study less than 1% of patients discontinued the drug due to adverse reactions.   But those solid trial results may not be enough to keep it ahead of Xtandi.

Accelerating growth

Zytiga faces stiff competition from Xtandi, a drug codeveloped by Medivation and Japan's Astellas.  Xtandi won FDA approval for mCRPC patients in August, 2012, and appears to be winning market share away from Zytiga this year.

In the third quarter, Xtandi sales totaled $108 million, up 32% from the second quarter. Meanwhile, sales of Johnson's Zytiga grew just 3% quarter-over-quarter in Q3. That dynamic may continue as Xtandi eats into Zytiga's international sales next year. Xtandi entered the fourth quarter with approval in 12 countries, and applications for approval filed in 35 more countries.

But Johnson may be more nervous about the FDA's pending decision on whether to expand Xtandi's label to include pre-chemotherapy treatment.  If the FDA grants that, it would eliminate Johnson's advantage as a second-line treatment in the United States.

Ongoing disappointment

While Johnson and Medivation's treatments have been growing rapidly, Dendreon's Provenge continues to disappoint.

Provenge, which uses patient's immune system to fight prostate cancer, won FDA approval in April, 2010, but never lived up to expectations. In trials, Provenge increased advanced prostate cancer patients' overall survival by four months.

But sales of the drug failed to take off and are struggling again this year, forcing Dendreon to lay off a significant portion of its employees, including members of its sales team.  Sales of the drug totaled just $68 million in the third quarter, which was below analyst's $75 million projections, and 13% less than a year ago.

Heading into 2014

2014 marks an important year for the three companies. But the year could prove most critical for money-losing Dendreon.  If Provenge sales stabilize, expense cuts could help the company survive. However, if sales continue to shift away from Provenge to Zytiga and Xtandi, then Dendreon's survival may be in question.

For Johnson's Zytiga, warding off challenges from Medivation's Xtandi may prove difficult; particularly given Johnson will face off with the drug in more markets. Additionally, if the FDA backs Xtandi in the all-important pre-chemo indication as a second-line treatment, Zytiga may lose share in the United States, too.