Silver
Silver reacts with the virus in a werecreature's blood, causing a rapid and dangerous rise in temperature.
The effects of silver in a wound are instantaneous. Even a thin coating of silver on a normal blade can do damage, but the silver must be nearly pure (sterling silver, high-quality silver jewelry, or unadulterated silver coins). The DM determines the purity of any silver - silver alloys may do only half-damage.
The mere touch of silver on unbroken skin hurts werecreatures as well. Any werecreature in contact with silver for a full turn (10 minutes) must make a saving throw vs. poison or break out in an ugly, painful rash (use the were-form's saving throws). This rash, accompanied by a fever, lasts 1d4 days for each turn the werecreature was in contact with the silver. All ability and skill checks, combat rolls, and saving throws are at -2 while the rash remains. Even if the saving throw is successful, the silver feels white-hot against the werecreature's skin.
Gloves and other clothing protect werecreatures from silver poisoning. The allergy cannot be cured without curing the lycanthropy as well.
Optional Rule: At 15th level, the effects of the rash are reduced to -1. At 25th level the rash is painful and irritating, but can be ignored with a successful Constitution check.
- but there's no mention of it in the SRD. 
