Are you asking because your ned has a non-pink muzzle that is badly sunburnt? It could be warts that then burn. Or it could be photosensitisation because the muzzle is hairless so more readily burnt. If it's a badly burnt, non-pink muzzle, I'd be suspicious. St Johns Wort and the dreaded ragwort both cause this and the problem can crop up years after the original ingestion.
I asked as Ellie had a sore spot about 5mm across yesterday. I thought sunburn but it was on brown skin not pink. Turned out shes pricked herself on barbed wire trying to get at grass the other side of the fence. Daft sod