Biotech investors are no stranger to a trading halt that brings game-changing news to a stock, but you don't see it very often when it comes to big pharmaceutical companies, especially ones with $200 billion market caps.

So it came as a surprise when Pfizer (NYSE:PFE) saw a trading halt earlier this week, and the cause was negative news for the company. A US. District Court decision invalidated Pfizer's patent on its blockbuster drug Celebrex. It is the fourth best selling drug for the company, as 2013 Celebrex sales reached $3 billion and are continuing to grow.

Generic competition for the drug could begin as early as May 30, which would be cause to celebrate for patients and payers, along with the generic-drug makers. But where does this leave investors in Pfizer? 

In this video, Motley Fool health-care analyst David Williamson discusses the court's decision, how this will affect Pfizer's sales, and why ultimately losing billions in revenue shouldn't change investors' thesis on the stock.

David Williamson owns shares of Pfizer. The Motley Fool recommends Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.