Can it be dangerous to suddenly completely stop taking Wellbutrin?

Wellbutrin, like many anti-depressants of its type, has a cumulative effect, which is why your doctor probably told you it would take up to 6-8 weeks before you would begin to feel the effects. The body essentially builds up a store of it.

If you abruptly quit taking it, your body will gradually secrete it until there is no more left in your system. So, no, sudden cessation should cause no harm at all.

Personal note: I have taken this (prescribed) drug. When I told my doctor I no longer wished to, he said something to the effect of "okay," and I just stopped taking it. No ill effects.