I think it should apply 4 or 5 ticks per time. This would do what mustard was suppose to do... deal damage over long time. You could ignore this and rush enemies and it would not change much, however if you were to be exposed on mustard for longer time, you would gather 50 poison ticks in no time. This would put a very heavy toll on your stimpack supplies. As for counter... antidote. While you are at it, i also suggest to buff antidote, to heal 50 ticks instead of 25.