The adder has one of the most advanced envenomation systems of all snakes, the vipers. Folding fangs and large venom glands compared to other species like cobras, mambas and kraits etc. How dangerous an adder bit can be depends on numerous factors to be honest, time of year, size of snake, amount of venom delivered, last time it used it's fangs/venom, number of fangs that make the strike, size of victim, health of victim, age of victim, location of bite and there is more. Drop for drop it is as potent as a Timber rattlesnakes venom and comprises haemotoxins, cytotoxins and neurotoxins, making it a very dangerous mixture. Fortunately for us, the adder is quite small and is not capable of delivering a large dose like the rattlesnake.
The adder can and does kill people around the world,but the last fatality here was in the 70s. A good solid bite is not pleasant and can affect people for months after the bite. Steve Backshaw was bitten in the ankle and had to spend a few days in hospital not so long ago, and that snake only got one fang in. I've been bitten twice and the affects of the second bite were worse than a bite I received from a large puff adder, and the pain unbearable for a good 12 hours, although I was very lucky with the puff adder as it only got one fang in, and it was one that was replacing main fangs at the time so I got a very small dose and the fang left in the wound. That adder bite was a large male in the mating season in Spring when potentially the venom is more toxic after maturing in the venom glands over the hibernation months and the snakes more bold and defensive as it was catually attached to the female I had picked up. It is very unpleasant.
Recent evidence suggests though, that it is quite rare for an adder to inject any venom in the first defensive bite and when venom is injected, they may only release a small dose as it is a precious commodity that is essential for catching prey and requires energy to produce. You would certainly be in deep deep trouble if you received numerous bites with full doses of venom, but this is almost unheard of. In fact I can think of only one case and that was on the continent in the Sixties.
The antivenom is very good nowadays though and you shouldn't be too worried if you do get a bite. It is certainly no bee or wasp sting as many people think and is far more potent and carries the same dangers that they present in terms of shock and allergic reaction. Small children and the elderly should get to hospital immediately to receive treatment.
