I was on Effexor for a few yeas-along with pretty much every other SSRI and SNRI. I had the same experience-if I missed a dose I knew within hours. Effexor has one of the shortest half lives of all the SNRIs and SSRIs, which means, the discontinuation symptomes (what the medical community calls it instead of withdrawal) are quick and bad for many people.

I see that you are in Canada. See if your psycharist (or MD) can refer you to something called The Impact Study-Through the Centre for Addiction and Mental Health in Toronto. It is free and I participated in it recently. They send you a kit where you do a cheek swab and a salvia sample and mail it to them (all pre-paid so it costs you nothing). They will measure your liver enzymes to find out what anti-depressants work best with your body and which do not work). It showed for me that I metablolize most medication extremely fast. What this actually means for me is that I end up experience discontinuation symptoms extremely badly-which may be the case for you since you are feeling it within hours of missing a dose. Have you ever tried to go off an SSRI or SNRI? I started going off Zoloft in May. I did a slow taper, then switched to liquid prozac (prozac has the longest half life of all drugs and is therefore easiest to go off) and tapered very slow in the end. Even with that, I have been off it a month now and still get brain zaps. Soooo not fun!
At least by participating in the Impact Study, I know that there's a reason why I experience this. So it may help you and your doctor to find out the best medication for you.

Here is a link to the website: IMPACT | Individualized Medicine: Pharmacogenetic Assessment & Clinical Treatment
Hope it helps
Misfit :-)