Is peer-reviewing a manuscript by a coauthor of yours truly ethical? Whereas it is often considered unethical to manage as an editor, associate editor or guest editor a manuscript from a researcher with whom you have coauthored one or more papers, this does not typically hold true for peer-review.

In other words, it is considered acceptable by most editors for me to review a manuscript from one or more authors with whom I have coauthored one or more papers, and even authors with whom I am currently collaborating.

But is this truly ethical?
When you are being asked to peer-review a paper authored by somebody you have collaborated with (or are collaborating with), you should be aware that you are on a slippery slope.

The very least you should do is:


Ask your self the question whether you think that you can provide an objective and unbiased review.
To inform the editor (or whoever invited you) about this.


There are situations where it might be acceptable, and situations where it is not. It is certainly not acceptable when you have a clear interest in the outcome of the review process (think of reviewing a grant application, where you would be benefiting somehow).
From the other hand, when you are (one of) the clear expert on a certain topic, and had only a superficial collaboration with the authors, it might be acceptable (in some narrow fields it might be kind of unavoidable).