Why? In Aegerion's case, executives blamed, in part, the weather.
"A particularly long harsh winter resulted in delays in outbound physician calls and a slowdown for patient onboarding," Craig Fraser, Aegerion's commercial head, told analysts on a conference call, noting the company was also re-aligning its sales force through January. "These factors contributed to the year beginning with a flatter script trend and thus, we had fewer new patient starts in the first quarter than expected."
The Cambridge, Massachusetts-based company sells the drug Juxtapid for a rare, genetic cholesterol disorder called homozygous familial hypercholesterolemia, or HoFH. It's characterized by extremely high levels of cholesterol that can cause kids to have heart attacks in their teens and often not survive past age 30.
The company reported first-quarter revenue of $27 million, missing analysts' $33 million estimate. It also lowered its 2014 sales forecast by $10 million to a range of $180 million to $200 million. While the stock was up on Thursday, it was recently trading down 19 percent for the week.
In addition to the weather, Aegerion also cited a slowdown in Brazil tied to an ongoing investigation into corruption allegations regarding prescription writing for the shortfall.
But weather also came up in the earnings' conversation elsewhere.
Regeneron's first quarter also came in short of expectations, sending the company's stock down in pre-market trading Thursday before it inched back into positive territory. The company reported U.S. sales of its age-related macular degeneration drug Eylea of $359 million, missing analysts' average $404 million estimate.
Read More Allergan's Valeant defense: Buy Shire?
What contributed? Distributors started using up their inventory, for one. And, for another, the weather.
"The weather in the Northeast and Midwest and Mid-Atlantic, it's obvious, it was horrible," said Chris Raymond, an analyst with Robert W. Baird, in a phone interview Thursday. "It's common sense, thinking about AMD, it's age-related, meaning it affects the elderly, so who's going to be perhaps less able to get to the doctor's office during bad weather?"
It was Regeneron's weakest showing for the drug since the third quarter of 2013, RBC Capital Markets analyst Adnan Butt wrote in a research note.
NPS Pharmaceuticals, maker of the drug Gattex for short bowel syndrome, dropped as much as 10 percent Thursday after reducing its 2014 forecast because of a "challenging January and February." Why? "Severe winter weather," CEO Francois Nader said in the company's release.
NPS now expects 2014 sales of $100 million to $110 million, down from a previous forecast of $110 million to $120 million.
But it may not all be gloom and doom. Baird's Raymond points out that delayed sales from the first quarter could provide a boost to the next one.
"Especially with Regeneron, I would be kind of licking my chops because I think this means you're going to see a rebound," he said. "You'll see some carry-through from patients who would have been treated in the first quarter, so we'll see bigger numbers in the second quarter."
And, hopefully, some sunshine.
-By CNBC's Meg Tirrell.