What happened Biopharma heavyweight Bristol-Myers Squibb (NYSE: BMY) saw its shares sink by a staggering 15.2% in 2018, according to data from S&P Global Market Intelligence. What caused investors to hit the exits last year? While biopharma stocks in general performed rather poorly in 2018 due to a bevy of headwinds, Bristol's stock was particularly hard hit due to its star immuno-oncology drug, Opdivo, losing significant ground to Merck's (NYSE: MRK) Keytruda over the course of the year. Image source: Getty Images. So what The long and short of it is that Merck's Keytruda was able to solidify its already-dominant position in untreated lung cancer during 2018. The net result -- and the issue that seems to have weighed most heavily on Bristol's shares last year -- is that Keytruda's win in front-line lung cancer should allow to it generate nearly $2 billion a year more in sales than Opdivo once these drugs reach their commercial peaks in the next five years, according to a report by EvaluatePharma. Two short years ago, though, industry insiders widely believed the opposite would be the case. Now what With Merck taking the pole position in first-line lung cancer with Keytruda last year, Bristol decided to kick off 2019 with the massive acquisition of cancer specialist Celgene Corporation (NASDAQ: CELG). This megamerger -- if it proceeds as planned -- would instantly transform Bristol into a top dog in the high-value oncology space. Celgene, after all, sports a rich pipeline of anti-cancer therapies, including next-generation product candidates such as bb2121 and liso-cel. That said, there are concerns that regulators might nix this deal -- or perhaps force some key asset divestitures -- over drug pricing concerns. As such, Bristol's stock may continue to struggle until this proposed megamerger actually comes to fruition. 10 stocks we like better than Bristol-Myers SquibbWhen investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has quadrupled the market.* David and Tom just revealed what they believe are the 10 best stocks for investors to buy right now... and Bristol-Myers Squibb wasn't one of them! That's right -- they think these 10 stocks are even better buys. Click here to learn about these picks! *Stock Advisor returns as of November 14, 2018George Budwell owns shares of Celgene. The Motley Fool owns shares of and recommends Celgene. The Motley Fool has a disclosure policy.