Er, do the Cullens use the restroom?

No
They dont the blood they drink get used up as energy

Because I dont like twilight you caused me to burst out laughing- Thanks. To answer the question I dont know or care.

I remember reading SMeyer's answer to this one somewhere in her site: no, they don't. She didn't explain where the blood goes, but neither did Bram Stoker or Anne Rice, for instance *shrugs*

perhaps it just dissolves and then when its all gone thats when they thirst for more?

Um, random! random! idk

No, but they get periods XD

Lol, another plot hole SMeyer overlooked...

Um I really don't know but in the books the never use the restroom. But you never know if they wear diapers or pampers or if they wet their couches or carpets(since they have no beds) or if they do their business on the floor or in a plant pot........but I really don't know.

To be honest, most fictional characters never use the toilet, whether they're human, vampire, werewolf, elf, hobbit, whatever...I think writers get a little too squeamish about it. Even in Harry Potter, the toilets were only used for brewing Polyjuice potion and taunting Myrtle ^_^

no they don't need to go to the bathroom

Meyer's vampires are clearly just straight up magic (like with the psychic powers and elemental abilites) so that's the easy answer; but that's boring. I like scifi and thought experiments so let's work through it. I think Meyer herself has explicitly said her vampires don't poop. For the question of "where does it go?", it actually could be exhaled. The "C" in the CO2 we exhale actually comes from the food we've eaten. As it gets broken down to smaller and smaller parts some of it eventually reaches individual CO2 molecules. It's plausible that the vampire digestive systems are so efficient they can break down their blood meals to gases that can be exhaled. In fact, that could explain why they may have horrible breath that smells of rotting flesh; they need to exhale gaseous sulfur and phosporous compunds. But, this doesn't apply to Twilight vampires specifically; Bella certainly never mentions it. Twilight vampires can clearly still breathe as they do to smell and I'm pretty sure it is mentioned most of them still breathe out of habit, but they can hold their breaths indefinitely. This explanation would require them to breathe at least sometimes but no where does it say Twilight vampires NEVER breathe. This could actually even apply to iron. Iron can bond to soluble organic compounds (afterall, that's how it exists in our blood without getting chunks of iron in our bloodstreams). So, iron and other difficult to vaporize elements or compounds could be sweated out or otherwise excreted as a liquid. Again, Meyer has mentioned her vampires don't sweat or pee but there is another way. Vampires could cough or sneeze. The dissolved iron could be expelled in tiny droplets just like how liquids are expelled as aerosols when we cough (like bacteria being scattered when we're sick). I'm really into scifi so I love these kinds of questions :).