DXM is primarily metabolised by CYP2D6 into dextrorphan. Poor metabolisers (~10% of Europeans, ~1% of Asians) experience dramatically stronger effects at the same dose — sometimes 4-5× potency — because the conversion is slower. Many common drugs also inhibit CYP2D6, effectively converting you to a poor metaboliser. If this is your first time or you take any regular medication, start at the very bottom of the 1st plateau range and wait a full 2 hours before redosing.