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What: Shares of biopharmaceutical giant Gilead Sciences (NASDAQ:GILD) took a breather from its monstrous run higher in 2014 and saw its share price tumble 10% in November, according to data from S&P Capital IQ, on the heels of three moderately negative events.

So what: Perhaps weighing on Gilead the most are slowing prescription sales of its hepatitis C drugs Sovaldi and Harvoni. As noted by Barron's, Harvoni scripts for the week ended Nov. 21 totaled 4,366, up 21% week-over-week, but total prescriptions including Sovaldi rose to just 7,813, a mere 5% week-over-week increase. Furthermore, difficulty getting Harvoni prescriptions written looks as if it could extend for a few more weeks. 

Secondly, Gilead priced a $4 billion debt offering in November. $3.5 billion of its $4 billion in debt isn't even due until 2025 ($1.75 billion) and 2045 ($1.75 billion), but investors clearly don't like the idea of Gilead potentially adding to its debt (even though this debt may simply be used to repay existing debts).

Lastly, per data from Yahoo! Finance, there's been some modest selling in Gilead's stock from insiders. November saw seven open market sales of various size, albeit some were simply options being executed and sold. Either way, investors might view this selling as a sign from Gilead's board that it's fairly valued here.

Now what: Although some headwinds do exist for Gilead, including getting insurers and pharmacy-benefit managers to include its $1,000 per pill Sovaldi and $1,125 per pill Harvoni on their approved medications list for HCV, I believe any significant downside in Gilead could represent a strong buying opportunity.

Source: Gilead Sciences.

Frankly, there's nothing quite like Harvoni or Sovaldi on pharmacy shelves as of yet. In just three years hepatitis C patients have gone from a standard of care that included IV interferon which often came with nasty flu-like side effects for up to 48 weeks and that had roughly only a close to 50-50 success rate in curing the disease, to HCV drugs that have a sustained virologic response of 90% or greater in most genotypes and that are administered without interferon.

In sum, Harvoni and Sovaldi are set up for immense success over the long run. Even if new competition enters the market (which is likely) it's not as if Gilead's patient pool is going to be dramatically affected. Gilead managed to treat only 117,000 of the U.S.'s 3.2 million HCV patients through the first nine months of 2014. Given that the worldwide population of HCV patients totals around 180 million, the moat for Gilead is still huge. Between these HCV drug and its four-in-one HIV drug Stribild, this is a monster company that any growth investor should consider adding to their portfolio.

Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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