Doesn't matter what drug you are talking about there is always a difference between subcutaneous (SC) and intravenous (IV) dosage.
The reason for the SC injections is usually for localised treatment. (eg. Local anesthetics) this is usually small doses given with adrenaline to hold the drug there for longer.
Systemic absorption from the skin layers is very small and slow. As said, intramuscular (IM) absorption is much faster but not as instant as an IV dose. Thats why IM injections are usually called depot injections because the drug slowly absorbs from the site.
IV is directly into the blood, therefore directly to the blood brain barrier, where the DMT can take its effects. I wouldn't suggest IV unless you are in a controlled research setting. Plus, the quality of dmt people extract in their backyards is not of good enough purity to be injected. You would need a pure batch from a lab to be safe.
However, the method for checking if you are in a vein or in the muscle tissue is to draw back on the plunger after inserting the needle into the upper lateral quadrant of the quadriceps. There should be no backward aspiration of blood into the syringe, if there is, pull out and try again.
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