I would strongly recommend not using straight syrup, I believe that DXM isn't very water soluble although I'm not positive. Anyways the additives in the syrup aside from the thickness of it I'd think you'd have a difficult time getting it to absorb easily. Sorbitol If I remember right has effects that could upset your stomach/bowels.
I think the best way (if you were to try plugging DXM) is to get pure powder whether it be from an extraction or painstakingly emptying Robo-gels through softening them with heated water or using a actual syringe and collecting the liquid and placing it in your cup/container; from each one, or even obtaining/purchasing bulk pure DXM from online vendors.
The thing is with plugging is that you don't use too much liquid otherwise you'll just feel like you've got the runs and either overdilute your psychoactive substance (dependant on ratio of mg's to ml)- or have a ridiculously slow absorption time. The walls inside your rectom soak up lower quantities of liquid and the drugs get into your bloodstream bypassing metabolism actions - having near the best bioavailability. So overall you'll most times need less of the drug to get just as high/higher (so use caution)
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Went to go check for solubility information.
(From
http://www.erowid.org/chemicals/dxm/faq/dxm_chemistry.shtml)
Table 3: Differential Solubility Data
Substance Cold H2O Hot H2O Ethanol Ether
DXM HBr Soluble (<1.5%) Soluble (25%) Soluble (25%) Insoluble
DXM free base Insoluble Insoluble Soluble Soluble?
Guaifenesin Slightly (1g/20 ml) Soluble Soluble Soluble
d-Pseudoephedrine HCl Soluble Soluble Soluble Insoluble?
d-Pseudoephedrine free base Slightly Slightly Soluble Soluble
Acetaminophen Insoluble Soluble Soluble Slightly
Propylene Glycol Miscible Miscible ? Soluble
Polyethylene Glycol 400 Soluble Soluble Soluble? ?