Biogen has a lot riding on aducanumab's success. In 2019, its product sales only grew 4.5% year over year mainly thanks to multiple sclerosis drug Tecfidera and spinal muscular atrophy therapy Spinraza. But the MS field's getting more crowded with multiple new entries, and Spinraza's facing Novartis' gene therapy Zolgensma, a potential cure. After Biogen recently cleared a patent challenge to Tecfidera, Goodman assigned a $410-per-share price target to the company's stock. That's significantly higher than the average $296.7-apiece average price Buffett paid for his stake. Apart from Biogen, Buffett's Berkshire also has been pumping money into Teva since a major shakeup initiated by CEO Kåre Schultz in late 2017. However, that investment hasn't exactly paid off, as the generic giant's newly completed overhaul has yet to translate into any major business improvement.