The barb at the end of a wyvern’s tail is covered in small pores that allow the wyvern’s poison to seep through. If a wyvern manages to cut you with the barb, the poison seeps into your bloodstream where it will eventually kill you. How long this takes and what other effects it has leading to death vary among species.
The poison glad is at the base of the barb. Some species of wyvern have an identical gland in their mouth and can inject you with poison much the same way a snake does though their hollow upper fangs.

There is no known physical cure, though powerful healers can neutralize the poison if it is gotten to in time.

Fever sets in shortly after the poison enters their system. 5 minutes later paralysis sets in. Blood begins to clot and the heart slows down. Death occurs ½ hour after the onset of poisoning from heart failure. If a healer is gotten to within the first 10 minutes, there is an excellent chance of complete recovery.

Paralysis is instantaneous. Healing is possible for up to 15 minutes afterwards. Death occurs after ½ hour from respiratory failure.

Poisoning can be stopped if the individual in question is a healer, but it is otherwise fatal within 20 minutes. They die from suffocation as they cease to be able to inhale

Paralysis begins to set in at the instant of poisoning. Time before full paralysis varies. Fever sets in as their body shuts down. Death is determined by the dosage of poison. A small dose leads to respiratory failure while a large does leads to brain death.

Paralysis is instantaneous. As the only way for a dragon to come into contact with wyvern poison is in combat, which occurs up in the air, this paralysis is fatal as the dragon will plummet to their death.