There are substantial ethical concerns in all forms of medical research including psychological research. How one approaches those issues depends on the ethical system in which you are operating. A consequentialist would argue, for example, that the question might only be answered in light of how the benefits gained might outweigh the harm done in each specific case. Deontological ethics would argue that certain actions are never justified, Most universities and hospitals have research ethics boards that monitor whether given experiments comply with ethical guidelines for use of human or animal subjects.
One typical question might be the degree to which a research on pain can be justified in inflicting pain on either student volunteers (who can give informed consent) or monkeys (who did not consent to being captured and tortured by researchers.) Some people would argue that both are wrong, and some that neither are wrong.
That would depend on the type of psychological research.