A million-dollar wound or Blighty wound is military slang referring to a type of wound received in combat which is serious enough to get the person sent away from the fighting, but is not fatal, nor will it leave the person permanently crippled.
In conclusion, Benadryl does not cause kidney damage directly, but if you uses excessively over a long period of time and there is urinary retention, the chronic urinary retention could ending producing kidney failure.
But, there are definitely both short-term and long-term effects of using diphenhydramine, the active ingredient in many antihistamines, (like Benadryl) that are worth noting.