Are you sure that it's the prepuce? Labradors seem to be one of the more common breeds to have a sheath that has an opening that is too small, which for some reason causes the penis to stay erect. It would be a bright red.

But, you could also be talking about prepuce inflamation which is a disorder and can be treated with pills. It isn't dangerous at all. Only this disorder comes with a yucky secretion.