Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of drugmaker United Therapeutics (Nasdaq: UTHR) plunged 17% today after its experimental orally administered lung drug failed in a late-stage clinical trial.

So what: Given how badly the results missed expectations, skepticism is naturally growing among investors over whether the drug will ultimately achieve regulatory approval. Wall Street had estimated that an oral version of treprostinal could potentially drive peak sales of about $750 million, higher than United Therapeutics' sales figure from all of last year, so it's no surprise that the shares are crashing as Mr. Market significantly lowers his growth expectations.

Now what: I'd look into this drop as possible buying opportunity. Given oral treprostinal's success in another trial and the fact that it is already approved in injectable and inhalable forms, management remains upbeat about the drug's prospects. While the chances of approval in late 2012 to early 2013 are certainly lower after today's news, United Therapeutics' stock price -- down 40% over the past three months alone and currently sporting a forward P/E of 12 -- might actually be safer after today's plunge.

Interested in more info on United Therapeutics? Add it to your watchlist.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.