home euthanasia

Well, it's getting close for my senior dog, and I'm really hesitant to take him to the vet for a final visit. I'd much rather have him go at home in the comfort of his own surroundings. The question I have is whether anyone can suggest a good, painless solution that is easily available to help let him down.
I thought over the counter codeine phosphate would work, but what is an appropriate dosage for a dog of 70 lbs?

please, no scolding replies, this is a hard enough thing to do without the reminders.