benadryl as preparation for euthanasia

Question
Due to terrible and debilitating neurological damage in my 8 year old beagle, I have decided to finally euthanize her this afternoon.
 She has to be carried almost everywhere because her brain won't tell her how to walk and her only enjoyment in life is
eating. Her legs are starting to have seizures and her quality of life is very minimal. I have used dilphenhydramine to calm rashes for her before and it makes her very sleepy. I know that it is also used as a sleeping med.  
 ??I would like to know that -- If I give her a double dose of 50 mg. 40 minutes or so before the procedure,will it put her more to sleep before we get to the vets office and also I want to make sure it will not interrupt or adversely affect the euthanasia drug so that this may be carried out swiftly.  The vet used to give a shot to make the dog fall asleep before the heart stopping drug was given, but they tell me they don't do it this way anymore and there is only one shot given.  I am leaving in 90 minutes or so, a swift reply is needed.  I have chickened out on this plan on numerous occasions and I have got to get this over
with for her sake. She is getting very tired and sometimes afraid.
Thank You.  Please answer.   Sue Hughes

Answer

Hi Sue and Tim,

I'm very sorry to hear that you are needing to put your beloved dog to sleep. It's never an easy decision, but it's one made from love. I don't know if you'll be getting this message within the time frame you asked, though.

You don't need to double the dose of Benadryl for it to have a sedation effect on your dog. The appropriate dose for your dog (25 milligrams which is 1 adult regular strength capsule) will normally make her a little woozy. The injection your vet will give her works amazingly fast. Your dog won't suffer or struggle, and her end will be peaceful and calm. Giving your dog Benadryl really isn't even necessary.

I hope I've been a help.
Patti