I take melatonin, not nightly, but maybe two or three or four nights a week. I have tried to read up on side effects and have not found much information at all. I don't notice any side effects, myself.
Since melatonin is a short-acting drug, and the therapeutic effects of it do not seem to last long at all, I'm thinking that any side effects would not last longer than a few hours or a day or two -- at most -- after you discontinued using it. So, if I were you, I would "run the experiment" and see if the dry mouth persisted after stopping melatonin for a few nights.
Over-the-counter products that are generic and that do not have to undergo FDA testing are hard to get the same kind of side-effect information about that you can find on prescription drugs.
I take melatonin almost nightly for chronic insomnia & have never noticed dry mouth before. But since this is a substance already in our bodies I wouldn't think that if it is a side effect of taking the supplement that it would last long at all. I would try going without it to see both how long the dryness last & if it goes away. Also since it is not regulated by FDA it could be caused by something else in with the melatonin. Just a thought.
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