I believe it's pretty dependent on the individual and their experience with the two substances. If you're used to both, they may go well together, whereas someone with little tolerance to nico and/or weed are more likely to experience light-headedness and possibly nausea. It really depends on personal preference and experience with the substances. I've got friends that love the combo and friends who despise it. I believe i'm somewhere in between. If you are planning on trying this combo out, i'd go with nico first, then the bud, mainly to get rid of the awful taste of cigs. If you're just vaping, my buds and I would almost always mix the two just by force of habit- and convenience.
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