This happened to me and I found good advice on the Wiki:
"In the case of deadly poisoning, asking a sage for healing is the only chance of survival. In such a situation, it is advised to beg the sage for healing multiple times, and to be awake 8 a.m. when the game's morning check happens. Even then, the odds of survival are only 1 in 3. One may try to improve these odds by praying to one's deity of choice.
Actually, it's possible to have much better survival odds than 1 in 3 - in fact, survival is almost guaranteed if treated correctly. If you get deadly poisoning, find a sage or shaman as soon as possible, and ask him three times to heal you (if the icon for deadly poisoning in the injury menu has not turned green after this, then ask again). Then, gather herbs that say in the description that they remove toxins from the body (Black Currants work as well). Until you recover, eat ONLY herbs with this effect. DO NOT eat anything else, as doing so will fill your hunger bar with foods that will not help, leaving less room for herbs that will heal you. If you do all this, you are almost guaranteed to be cured within a couple days as long as you do not have any significant injuries (deadly poisoning may raise your injury rating above 50% before it goes away)."
I would add that some herbs such as bear pipe increase appetite, allowing you to eat even more herbs that reduce toxins.
Best advice is to neither eat, carry nor store any mushrooms unless you are 100% certain they are edible... for what you get from them it's just not worth the risk of an accident!