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Never tell someone that they are --going to have a bad trip-- just because you had one bad experience. Many have blended those very same elements together and have glowing testimonies. It is, as Jorkest says, dependant upon the person. Not to mention setting,etc.
DNA damage? Cite evidence.
As for the mix, Easy on the Caapi Copy. It does not take much. I would first TRY the mixture in a bowl. If you are pleased with the ratios, go from there. But mixing changa on a whim or even advice from someone here without testing it first is unwise.
When vaping RIMAs(harmalas are not MAOIs, they are RIMAs), a little goes a long way. Put it in the bowl first, under everything else, then herb(dank, parsley, calea(yuck!, but nice), damiana, lotus stamens, mugwort, mullein, etc.) then spice and then more herb on top. Light the herb and let the embers do the melting/vaping of the stuff below.
While I personally do not care for harmalas without THH, you won't necessarily have a -bad trip-. Unless someone plants a seed in your mind that it will.
Experiment, start small, minimize waste, be safe, learn more about yourself than you ever knew and have fun,
Peace,
J