Hi, welcome to the forum!
Vaping DMT can take a bit of time to get used to. To develop a proper technique, nothing beats practice. I can't say whether or not codeine would help with your throat but from my experience, taking other substances on top of DMT can have positive or negative effects. It depends on the substance, the person taking it, the set/setting, and likely other factors. If vaping out of the glass sherlock doesn't work for you, you can always check out other methods of ingestions.
As a general rule, if you start taking syrian rue or other MAOI/RIMAs, be sure it doesn't interact negatively with any substances you do plan on ingesting. I'm sure you already know most or all of what I mentioned so I wish you well on your journey and hope you are able to remedy your vaping issues!
-TGO
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