After you flush something down the toilet, you poop as if you want to chase what you have just flushed down moments ago. This poop is referred to as a "chaser".
(Mr.Krabs flushes Plankton down the toilet)
Plankton: I'll get you Krabs!!!! glub* glub*
Mr.Krabs: Heh Heh, and now for the chaser.
A substance that is mixed in with drugs during preparation before seller them on the street to not only weaken them but stretch the amount of drug available in order to be sold to more customers and make more money.
The buyer rarely has any way to measure how much drug they are actually getting, it also poses a threat because they could be getting more drug than used to in addition to dealers chasing with more harmful substances or other drugs like angel dust.
Heroin and cocaine have been known to be chased with crushed aspirin when sold in powder form since it matches, the buyer rarely has any way.
"She took the smackstraight from the cooker without a chaser and overdosed."