Shares of Mallinckrodt (MNK) , a UK-based pharmaceutical company, plummeted over 35% in after-hours trading on Tuesday after it reported disappointing third-quarter results while offering a grim outlook.

Shares continued their decline during Wednesday morning trading, lower over 3%.

With Mallinckrodt's stock stumbling to an all-time low, here's a look at the three main reasons why investors have jumped ship.

1. Third-Quarter Revenue

Although Mallinckrodt posted adjusted earnings of $1.97 per share which topped analysts' estimates for adjusted earnings of $1.81 per share, its revenue came in under expectations.

The company reported sales of $794 million, representing a roughly 10% decline from last year, and missing analysts' projections for $807.8 million.

2. Warning of Slower Sales of Acthar

Acthar is the crucial reason for the stock's overall decline. Sales of the drug, used to treat infantile spasms and multiple sclerosis, slowed during the quarter due to written prescriptions going unfilled, Mallinckrodt said.

Acthar net sales declined in the quarter to $309 million, a drop of 6% year-over-year.

The drug represents Mallinckrodt most significant source of revenue, accounting for 42% of its overall revenue. And, the company said it expects further sales declines of the drug in the fourth-quarter vs. the third-quarter.

3. Drug Pricing and the Opioid Crisis

Another critical reason for the drug's sell-off may be that the outlook regarding Acthar's steep pricing could be in jeopardy.

The drug has come under scrutiny in medical journals, arguing that its effectiveness does not command the high price for which Mallinckrodt sells the treatment.

But it wasn't just Acthar which saw declining sales at the comapny. Sales of opioid-related drugs including Hydrocodone (API) and hydrocodone-containing tablets as well as Oxycodone (API) and oxycodone-containing tablets shrunk as well.

Hydrocodone saw a 42% decline in sales year-over-year, and Oxycodone saw a roughly 38% drop. These declining sales come as the U.S. takes steps to eradicate the opioid epidemic plaguing its citizens.

Making matters worse, in August Mallinckrodt was issued a subpoena by the Justice Department for documents "related to the marketing and sale of the company's opioid products."

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