Harry Potter and Viagra have more in common than you may imagine. They came to market within a year of each other in the late-1990s; they enjoyed enormous success; and what was a boon for the companies that produced and sold them could turn into a bane as their popularity fades and rivals emerge.

Publishers dream of discovering a new mega-seller; pharmaceuticals companies yearn for more blockbuster drugs. They should grasp these exciting opportunities if they arise but also remember to work out how to handle the medium and long-term consequences.

Nigel Newton, founder and chief executive of Bloomsbury, which published J.K. Rowling’s seven-book series, says he was well aware from pre-Potter days that unexpected hits were “as likely to destabilise the company as to propel further success”. Even so, when the company issued a profit warning in 2006, providing, in the words of one analyst, a glimpse of “life after Harry Potter”, the shares tumbled. They have yet to revisit their previous peaks.