Meant to propel growth, Daiichi Sankyo's foray into generic drugs by acquiring a majority of Ranbaxy Laboratories has instead erased four years' worth of profits. Daiichi has now taken a $3.5 billion goodwill writedown on its controlling stake in the Indian company — an investment that cost $4.9 billion only last year. The Japanese company's fresh financial forecast envisions full year net profit of $400 million; this is less than half of what it earned in fiscal 2007. On Monday, the president of rival Takeda Pharmaceutical said he's interested in deals in India and China. Daiichi's fiasco should provide a valuable lesson to its rival's boss. […]