Oral Benadryl Not Working Anymore; Extreme Itching

I unfortunately am no longer responding to oral Benadryl (EDIT: All forms that are OTC from capsules to liquid children's) but I have to take it right now because I'm in a really bad flare that has put me in the ER four days in a row. I have gastroparesis so I don't get it in me fast. I'm embarrassed by it but I have to go if I'm basically scratching my skin off and my voice is hoarse and I'm throwing up. I do respond to IV Benadryl, though. Has anyone done an IV Benadryl drip at home or had to administer doses of it themselves? I have a IV chest port so it would be extremely easy to get that accessed and then just have it changed out once a week.

I hate IV Benadryl. It makes me feel weird and it makes me cough when it's given no matter how slowly. But we can't think of anything else and I don't want to buy a time share at the ER. So this isn't me seeking anything but relief.

I hate everything about this. And I'm usually an upbeat, chipper person.