Exaptation, the emergence of latent functionality in existing artifacts, is an underexplored mechanism of novelty generation in innovation. In this paper, we measure the frequency of exaptation in the pharmaceutical industry. We find that about 42% of new functions derived from existing drugs have an exaptive nature. We think that this constitutes the first measure of exaptation in any industry. We also link exaptation with radical innovation and find that most radical innovations in our sample are exaptive. Also, nearly all radical innovations occur in market areas very distant from the drug's original market. We propose that exaptive innovation constitutes a different search mechanism and problem-solving approach from deliberate innovation and discuss the role of context and serendipity in innovation.