Well, not entirely. There's actually some work suggesting that we may influence some of the content of some recurrent unpleasant dreams. But recurrent nightmares usually suggest both that ones sleep chemistry isn't functioning well, and that there are significant issues roubling one, which may need to be dealt with, such as within proper counselling. Some medications, including some antidepressants but also a number of other meds taken for all sorts of reasons, can increase dreaming and, potentially, also increase nightmares