DXM isn't something you need to make stronger... As far as lasting longer, it already lasts pretty long, by itself. There are two forms commonly available in the US: DXM hydrobromide, and DXM polisitrex... my understanding is that the polisterx, being delsum (or however that's spelled), is a plastic that somehow covers the DXM molecules, thus they take longer to metabolize. I wouldn't advise Polistirex on a first go... Iv'e done it once, and enjoyed it, but would have been way too intense for my frst time. DXM was my first psychedelic, at age 14. Be safe and read up. You can teach yourself anything, these days