The required voltage may vary 100:1 depending on circumstances but the share of current passing through the heart is always in the neighborhood of 30mA. So, current is a much more reliable measure of potential lethality: any source capable of delivering at least 10mA with enough voltage to deliver it on-contact is potentially lethal. A 5kV source that can only provide 10mA will cause local burns but is unlikely to be lethal.
For the record.. When I was 14 I was making a Jacob's Ladder, and I was using an old oil-burner ignition transformer. I've just done some research and see that some are rated at 20 or 30mA... the one I had was only 10mA.. I remember that. 10kV , 10 mA.
When I was building the Jacob's ladder, I accidentally touched the leads with both hands. I don't know how I came in contact with the wires, but it did throw me backwards about 5 meters into a wall, and I survived. I consider myself lucky, indeed.
In my case, 10 mA was survivable. I also now know that DC tends to throw one away from the source, and AC tends to make one hold on. But this was an AC transformer, and I did not hold on.