I am a medical student working on my thesis... I found a couple of theses online that have relevant info to my thesis subject... I took this information but rephrased them with my own words and then I copied and pasted the references from those theses! Is that ok? If not, can I cite those theses? Or what should I do at all?
Yes, you must cite works you use in your thesis.
Failing to do so is plagiarism. Also worth noting: copying and pasting work you yourself have written but for a different publication is also plagiarism (see self plagiarism).
I would argue that one of the main goals of a thesis is to demonstrate that you are capable of independent research by finding, critically analyzing and synthesizing existing information.
Showing you have done research is part of the point, you show that by citing other publications.
EDIT The original comment was edited with more detail very quickly after I started answering; let me elaborate.
Broadly, you should cite anything you use in your work that comes from another author. If you mention another author in your paper, your examiner (and other readers) should be able to find the work you're referencing in your works cited. Think about where the ideas come from and how you're using them.
More concretely, if the other thesis itself synthesizes works of many people and you are only interested in the broad conclusion, you could cite the thesis.
Suppose the thesis author cites Bayes in their thesis, and you write something like
thesis author (2010) is not in total agreement with Bayes (2008)*
thesis author (2010) is not in total agreement with Bayes
you need both in your works cited, because that's what you cited. You don't want to leave the reader (examiner) saying "who's this Bayes character?"
Do you have to read all the works the thesis author cites? Very likely not, but it's probably a good idea to read the foundational works the thesis cites in order to understand the thesis itself.
Conversely, as Guillaume has pointed out, don't cite a work you aren't familiar with (meaning: if you need to cite it, familiarize yourself with it).
You should at least have a big picture idea about each and every reference you cite. Here is why:
- Citing a reference without having read it will get you in trouble when you defend your thesis, for example if somebody asks what a reference is about, and you answer wrongly because you did not read it (while they did). This happened to me with my master thesis (and the professor who asked me told me later that it had happened to him when he was a student). Never again. :-)
- At least in my field, you can spot such "never-read-anymore-but-important-so-people-cite-them" kind of articles in the literature, and it's interesting to read them and see how their meaning drifts as other people cite them without reading them. This is another important reason to go read your refs: the work you found them through might have cited them wrongly in the first place!
And of course, as already mentioned, you must always cite everything, i.e. the work you found interesting and the references cited by this work that are also relevant to your own work. When there are too many such "second-level" references, for example because the primary article you cite is a review, you can also cite this way:
for review, see Doe et al (2010) and references within.
But use this with moderation; it is too easy to cite reviews without ever reading primary research articles, with the risk pointed out in point 2 above.