Q: When can I add a new citation to the manuscript, during the under review stage or at the page proof stage?

Congratulations on the acceptance of one of your papers! Please note that journals differ on their policies regarding the citation of unpublished papers; a paper that has been accepted has still not been published. Some journals allow the citation of accepted manuscripts as follows (with the exact format depending on the journal’s style):

In the text: Author Surname (in press)

In the reference list: Author Surname, Initial(s). (in press). Title. Periodical or Journal Title.

Avoid citing a paper that has been submitted or is under review. If it is absolutely essential, you could cite the findings and include “(unpublished),” but certain journals discourage this practice.

The best time to make the change in a citation and the corresponding reference in the reference list would be after peer review, i.e., when you resubmit the manuscript with the suggested revisions. However, in your case, it appears that the acceptance of the other paper happened beyond this stage. Therefore, you could make the change at the page proof stage, which is a good time to update any citations of submitted manuscripts that have progressed to the in-press stage or in-press manuscripts that have moved to the published stage. You should also write to the editor informing him/her about this change. Depending on the policy of the publisher, a fee may or may not be charged for this alteration. An important pointer: be very careful that the added citation and reference are error-free, as there will be no further proofing. Any error introduced at this stage would end up getting published!

You may have a look at a similar question posted on the forum: Can I correct an error in citation at the page proof stage?