The poison generally contains neurotoxins that damage the nervous system, or toxins that block blood clotting and therefore cause bleeding.
That of the viper, for example, contains toxins that inactivate the coagulation factors present in the blood: it is a complex of enzymes capable of destroying proteins. To these are added bradykinin, which causes the dilation of blood vessels and thus the drop in pressure, as well as substances that damage the nervous system and muscles.
The effects are therefore heart failure and hemorrhage. The poisons of elapids (cobra, bungaro, Australian snakes) instead contain powerful neurotoxins that damage the central nervous system and substances that destroy red blood cells.
There are no preventive antidotes that cancel the effects of a possible bite. Instead, there are specific serums for the different poisons.