Until your opponent has a press blitz going.
Disrupts the timing and puts too much pressure on the QB to be reliable. Slants are timing routes, so jamming the receivers can really disrupt it.
Problem is that it's a gamble like all blitzes. If they miss the jam they'll be horribly out of position and if the QB connects it's a 20+ yard gain. There's some zone coverages that are effective too, but you really have to commit to stopping the slant for it to work (Note: this is an excellent idea against the Eagles when I'm playing them.
I run quick slants from 3-4 different formations and hot route them often because they are a PITA to defend against).
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"But there's no sense crying over every mistake,
You just keep on trying till you run out of cake"