Friday's session closed the week on a down note for the stock market, with early gains being erased after a renewed FBI investigation of Democratic presidential candidate Hillary Clinton made investors more anxious about the uncertain fate of the 2016 elections. Yet even before that news sent the market lower, Amgen (NASDAQ:AMGN), McKesson (NYSE:MCK), and StoneMor Partners (NYSE:STON) were falling substantially. Below, we'll look more closely at these stocks to tell you why they saw such large declines.

Image source: Amgen.

Investors ignore Amgen's strong earnings, focus on Enbrel worries

Amgen fell 11% despite posting relatively strong financial results in its fiscal second-quarter report. The biotech company saw sales rise a modest 2% to $5.81 billion, and earnings of $3.02 per share were far higher than the consensus forecast among investors. Yet most of those following the stock focused on an increasingly cloudy picture for the company's plaque psoriasis and rheumatoid arthritis drug Enbrel. Amgen will have to make pricing concessions in order to ward off potentially even worse impacts from generic competition, and as a result, some investors fear that the biotech giant will struggle to find ways to replace lost revenue and earnings in the long run. Whether that warrants a double-digit percentage move downward isn't clear, but it's a problem that other biotechs besides Amgen have had to deal with lately.

McKesson takes a big hit

McKesson plunged 23% after releasing its third-quarter results Thursday night. The pharmaceutical distribution company posted weaker earnings and revenue than investors had expected, but the real problem lies in McKesson's future. CEO John Hammergren said that adverse drug pricing trends in the industry are likely to have a negative impact on the company's results in the near future, and decisions among McKesson's customers to price drugs differently could also hurt its own profitability. With election results more uncertain than ever, McKesson could potentially face an even tougher environment in 2017 than it currently expects.

StoneMor Partners punishes investors

Finally, StoneMor Partners cratered, falling 45%. The master limited partnership focusing on funeral and cemetery services said late Thursday that it would reduce its quarterly distribution by half, paying out $0.33 per share. StoneMor pointed to the need to use capital to boost its sales force, and CEO Larry Miller said that the MLP estimates that it could take six to nine months to get new hiring and training to a level at which StoneMor can evaluate whether returning the distribution to higher levels makes sense. With the distribution cut being roughly in line with the stock's percentage decline, StoneMor's yield will continue to be near 10%, but investors have to be nervous about how quickly the company can recover from its current difficulties.

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