If a bolus injection is given into the tongue, where does the drug PRIMARILY drain?

The tongue drains venously via the lingual vein → internal jugular vein (and then to the subclavian vein and superior vena cava). A bolus injection into the tongue would primarily drain into the lingual vein, which empties into the internal jugular vein. This is why intravenous drug injection into the tongue (or sublingual IV access in emergencies) delivers drugs into the systemic venous circulation rapidly via the internal jugular.