I've never had Nyquil, but if it has anything in it other than DXM, you should not exceed the recommended dosage. 

Especially if it has acetaminophen in it to help with pain, as this could be a [b][u]fatal[/u][/b] combination.

If you [i]really[/i] need a DXM fix, Delsym contains only DXM.

The only thing is that most cough syrups contain a ton of sugar, which could wreak havoc on your pancreas if you use them a lot, and DXM will make your mind a little slower for a while. 
Like, months.

Also your pupils are going to be huge for several hours.

If you start feeling really short of breath, or get shakes, and chills, then you should go to the hospital.