Naltrexone is an opioid antagonist. There are other simple options, like chamomile (or apigenin in it). One can contrast this with menthol (as in mint oil), a strong agonist. Chamomile usually helps, however I don't see any point in taking it regularly. It downregulates (kappa) opioid activity both in antagonism and afterwards, however at some point it just acts as a depressant with no additional benefit (but possible upregulation thereafter).
Menthol (the agonist) is a very telling contrast. It gives me some of the most intense POIS - however not before making me slightly euphoric. I have found Chamomile might not be enough afterwards, but when chamomile wears off after an intense menthol session, one still notices that menthol feeling in the head (similar to mentholy cough drops...), and it only goes down after an additional session of extra-high menthol (mint-oil), with some chamomile thereafter.
Bottomline: kappa opioid antagonists work, but don't seem to make sense longterm. Kappa agonists cause intense POIS symptoms.
(If you find thst too confusing, I haven't really said much at all, just confirmed the principles of antagonism and agonism, with antagonism being more beneficial or what one wants.)
