While perusing the S&W Whitebox rules today, I noticed that the Adventuring Gear section lists both belladonna and wolfsbane. Neither item is described, nor does the lycanthropes section mention either herb. In fact, the rule book never mentions either herb again.
So, I surveyed our family of rules to get a consensus on belladonna and wolfsbane.
Swords & Wizardry Core doesn't mention belladonna. Core's equipment description for wolfsbane says it will "often keep werewolves at bay... temporarily." The monster entry for werewolves says, "wolvesbane keeps them at bay." The lycanthropes entry in the Monster Book doesn't mention either plant.
(Incidentally, I haven't spent much time with the 4th printing of Core, but I like what I see. It's an improvement over the third printing.)
Labyrinth Lord (and OEC) lists wolfsbane, but not belladonna. It's noted as a ward against all lycanthropes ("the lycanthrope must succeed in a saving throw versus poison or free out of fear"). LL Advanced Edition includes belladonna, but only as an herb "used to relieve aches and pains, reduce inflammation, relieve coughs or used as an anesthetic."
Holmes includes wolvesbane in the equipment list, although makes no mention of how it affects were-sharks and other lycanthropes.
Moldvay lists only wolfsbane. The lycanthrope entry says, "if a lycanthrope is hit by wolfsbane, it must save vs. Poison or run away in fear. The sprig of wolfsbane must be swung or thrown as a weapon, using normal combat procedures." Moldvay doesn't mention any curative properties for those infected by lycanthropy.
The AD&D PHB lists wolvesbane and belladona [sic] in the equipment lists. The DMG says that when a character who has been bitten by a lycanthrope "eats any belladonna within an hour after being bitten, there is a 25% chance the disease will not manifest itself". The Monster Manual lycanthrope entry reiterates that. The only DMG mention of wolfsbane is as a treatment of last resort along with belladonna:
If the adventurer decides to be cured and the methods mentioned thus far have been unsuccessful, he or she may take refuge in a holy/unholy place such as a monastery or an abbey. There the clerics can administer to the afflicted one holy/unholy water laced with a goodly amount of wolfsbane and belladonna prepared by the spiritual methods of that particular religion. This potation is to be consumed by the victim at least twice a day from a silver chalice. No adventuring may be done by the character while he or she is being treated by the clerics. After a month or more (depending upon how advanced the disease is) the player character should be cured and somewhat poorer in the purse, as this procedure is very costly.
OSRIC doesn't include belladonna or wolfsbane in its equipment list, but the lycanthropes entry says: "if a victim ingests belladonna within one hour after the attack there is a 25% chance the disease will be cured." It makes no mention of wolfsbane.
Men & Magic lists wolvesbane and belladona [sic] in the equipment lists, but the lycanthropes entry in Monsters & Treasure mentions neither. The supplements don't mention either plant.
The d20 SDR says that a character afflicted by lycanthropy "who eats a sprig of belladonna (also called wolfsbane) within 1 hour of a lycanthrope’s attack can attempt a DC 20 Fortitude save to shake off the affliction."
Outside the game, wolfsbane is properly known as Aconitum, a genus of pretty flowers in the buttercup family. Atropa belladonna is a flowering shrub from the nightshade family. Though unrelated, both wolfsbane and belladonna are toxic---the roots in the case of wolfsbane, and the leaves and berries in the case of belladonna---and both have medical uses.
My very cursory research didn't reveal any strong folkloric associations of either plant with lycanthropy apart from twentieth century pop culture. (Flying ointment and twilight sleep are interesting.)
So, depending on your rules and your referee, wolfsbane might drive away lycanthropes, and belladonna might cure a character infected by lycanthropy if administered within an hour of infection. But they might not do anything. I sort of like that uncertainty, and I'm inclined to let the efficacy of belladonna and wolfsbane remain uncertain for players and their characters.