Typically scientists try to reject the null hypothesis (in this case, that a particular pair of drugs do not interact) by showing that it's unlikely (less than a 5% chance) that an apparent interaction is caused by random chance. But if you go looking at millions of possible interactions, you're going to find a lot that seem to meet that criteria by chance. 5% of a million is 50,000. 0.1% of a million is 1,000.

That's why drug companies have to prespecify the outcomes they're looking for before starting the study. If you look through your data after the fact, you're going to find something that makes your drug look good just by chance.