Can i take my second dose of zyrtec (cetirizine) d in 8 hours if i need it, opposed to the recommended 12 hours? took it at 10am would like to take it now 7:30pm

Yes: Turns out that twice daily Zyrtec D (5mg cetirizine + 120mg pseudoephedrine) has half the cetirizine (10mg) as plain Zyrtec which is dosed once daily. So we know it's not a cetirizine issue. Most likely it's b/c you don't want too much pseudoephedrine in your sytem at any time or else you might feel jittery & wired. Let's put it this way, you're probably safe at 8hrs but if you notice side effects

Should be Ok: try then to change to the recommended schedule as soon as it seems to be working