It's a big night for investors in both Amarin (Nasdaq: AMRN) and Horizon Pharmaceuticals (Nasdaq: HZNP), and for the companies themselves. Getting a drug approved is a major milestone.

After hours, the FDA approved both Vascepa (formally AMR101) and Rayos. Let’s tackle Horizon first.

Horizon’s Rayos will be its second approved drug behind Duexis, both of which are for rheumatoid arthritis, a huge market. The FDA also opened up the drug for use in COPD, asthma, and psoriatic arthritis. Horizon recently signed up a subsidiary of Covidien (NYSE: COV) to distribute Duexis, and it wouldn’t be surprising to see that deal extended to Rayos. If the drugs gain traction, it may make Horizon an interesting pick-up for Covidien.

Amarin’s triglyceride fighter, Vascepa, is highly-refined fish oil that aced both of its FDA agreed upon phase 3 trials, meaning approval was about as sure of a thing as we get in this business. Amarin is still waiting to hear whether Vascepa will be granted New Chemical Entity status, giving it a five-year window of exclusivity versus just three years of new product exclusivity. The company’s patents run until 2030, and its superiority to GlaxoSmithKline’s (NYSE: GSK) near billion-dollar drug, Lovaza, means Amarin should succeed, even after Lovaza goes off patent.

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David Williamsonowns shares of Amarin Corporation, but he holds no other position in any company mentioned. Click here to see his holdings and a short bio. Motley Fool newsletter services have recommended buying shares of Covidien. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.