Hi Nas,
Since nothing is clear about the cause of POIS, it is hard to tell what supplement will help you, so you have to test them safely, one by one, in order to know what will be of help for yourself.
If Dr Waldinger is right and there is an hypersensitivity reaction at the base of POIS, this imply a massive production of histamine in the body.. Thus, in this context, DAO should be useful in order to help the body getting rid of histamine excess ( DAO is one of the main enzymes that metabolises histamine in the body). This is theory, and reality is not always like predicted in theory, of course.
Do you have food intolerance? Do you have headaches ? Those are symptoms that are told to be associated with histamine intolerance, which is the main reason why people try DAO supplements. If you have these, it may be of help. For POIS, one member has reported good results, others said it was not of any help, so like any other supplements talked about on this forum, it helps some, and not others, and there are no predictable factors yet telling what supplement would be effective for what type of POIS. Everyone has to take its own decision to try or not a specific supplement ( as usual, by consulting before with a health professional, and using safe dosage)
However, when I first arrived on this forum, I have tried to a chart that could allow some predictions, by noting what supplement has been effecive for what member, and for what symptoms. After hours and hours of work, I realized it was quite difficult, mainly because what is shared on the forum is rarely cleared and detailed, which is normal, because it is not scientific reporting here, just members sharing what they have tried. However, I did notice that niacin tend to be effecive for those who have what I call "cluster 2" symptoms ( see my classification of symptoms at
http://poiscenter.com/forums/index.php?topic=2027.msg15927#msg15927 ), but there are exceptions, of course. So those who have mainly brain fog, cognitive symptoms, muscle twitch, muscle weakness, dermatological symptoms, and other pellagra-like symptoms like digestive problems, tend to have some level of relief with niacin. If you have such symptoms, niacin may be of help for you.
In any case, I wish you luck with your testing, and that you hopefully will find some relief with at least one of these two !