It's not the voltage, it's the current.
A general rule of thumb is anything even close to 1 Amp is very, very bad.
I believe I've heard that only a few milliamps, measured at the heart, can screw you up.
Also depends on what part of the body you get the electricity to flow through. And whether the current travels only on the skin, or actually through the organs inside.
Heard a story (urban legend?) of a dumb guy who nearly killed himself by trying to test his body's internal electrical resistance with a simple voltmeter. He kind of didn't remember that a voltmeter checks voltage by sending out a bit of electric current, usually supplied by a simple battery, which is normally harmless, BUT
he jammed the probes into his thumbs, giving the current direct access through his wet, slimy, conductive insides, from one arm, through the heart and out the other arm. A small but deadly zap.
Hunt down the Mythbusters episode where they did the toaster-in-the-bathtub experiment. Lots of details.