For a while, it seemed as if Eli Lilly (LLY 0.21%) could do no wrong. Its shares skyrocketed. The company's market cap grew so much that it was within striking distance of the $1 trillion mark. Along the way, Lilly became the largest healthcare company in the world based on market cap.However, the situation isn't so rosy for Eli Lilly now. The big pharma stock has fallen nearly 20% below its peak set last year. Some dark clouds hover on the horizon. Should you buy Lilly on the dip?Behind Lilly's declineTher ...