anything not from a prescription (yes you can get a zero strength script) can be dangerous. most are made in china and can really mess up your eyes, be cautious. I recall last year something in the news about these halloween lenses but cant seam to find it.
I am an ophthalmologist and would recommend against it. I don't practice what I preach however as I have worn them in the past. If you do wear them get a name brand disinfecting solution to soak them in before you wear them. When you remove them again soak them in solution. If they ever become uncomfortable take them out immediately.
You should at least get FDA approved lenses. Many places will not sell you lenses without a prescription even if they are not modified lenses. THis is to protect themselves from law suits. Wife got her eyes examined for lenses and then tried to order Halloween contacts on line and sent them the script. They told her she needed a script specificially for the Halloween lenses and would not honor the basic contact lense script.
Since this will probably pop up in the years to come when new members do a search, I'd like to add something. A prescription does two things: it tells you the strength of the corrective lens, and it tells you the shape/curvature of your eyeball. You need a prescription for Halloween lenses not to help you see but to make sure the lens will fit on your eyeball. Do you really want a pair that aren't the right size digging in to your eye?