The investment firm also slashed its price target to $297 a share from $360. The new target indicates 1.4% downside from the stock's Monday closing price of $301.26.
The firm says the Cambridge, Mass., company has "two major controversies" on the horizon: Tecfidera's intellectual-property challenges and aducanumab's potential approval in Alzheimer's treatments after the Biogen drug showed mixed results in a recent Phase III trial.
Analysts at J.P. Morgan expressed skepticism about aducanumab in a separate note, in which the firm reiterated a neutral rating on Biogen shares.
"Despite Alzheimer’s coming back into the picture post futility analysis, we remain skeptical as to approvability at this point and continue to believe it will be important for BIIB to build the perceived valuation of the company beyond [multiple sclerosis/spinal muscular atrophy/Alzheimer's]," analyst Cory Kasimov said.
Asked about the efficacy of aducanumab, 43% of polled physicians had a best-case view of the clinical data showed by tests of the drug. But the firm says an equal number of physicians do not believe that the data are clinically meaningful.
"While the company has materially increased the number of new-drug candidates in its pipeline, we do not believe investors are yet willing to ascribe much value here," J.P. Morgan's note said.
"Rather, many investors appear to be far more focused on the company’s approach to business development, where Biogen has been relatively conservative in the past."
Biogen shares at last check fell 2.8% to $292.89.