Journal articles may be retracted for different reasons, such as issues in authorship or data, conflicts of interest, errors in methods or analysis, falsification, and plagiarism.
Scholarly publication, theses and dissertations should not cite retracted papers.
The database Retraction Watch lists over 100 reasons for retraction. For example, this 2021 paper on material science was retracted by the journal editor due to the authors use of similar images in multiple articles.
According to Retraction Watch Database, every month hundreds of publications receive “Expression of Concern” or are retracted. Retracted papers are still being used; many researchers continue to cite them long after they are retracted. It is legitimate to discuss a retracted article in your work if you are aware of the retraction and the reasons behind; however, if you cite a retracted work without knowing, it may cast doubts on your research quality and integrity.
Adding the text “Retracted” at the title may be good enough for human readers, but it is not a smart way to communicate with machine readers. The metadata of the article should also tag retraction status. If the publisher uses CrossMark, a service by the DOI agency Crossref, you can see the information after you click the CrossMark icon.
Retraction Watch is a blog compiling reports on research integrity and retraction. The Retraction Watch Database has become the most relied-upon information sources of publication retraction. A very useful feature is their index of “reasons”; you can easily see why the papers were retracted
Scite.ai is a database that shows you citation contexts of publications. It has a function called Reference Checks, with which you can check if a list of references are cited as supported, disputed, or are retracted. You can upload your own paper to let it run through the references. Scite does not use Retraction Watch data, but relies on other data sources and their own algorithms. Therefore, we may expect that its performance in detecting and flagging retracted items would be different from tools that are based on Retraction Watch Database.