What antihistamines to take?

Hi all, I’ve had eczema come back for the past year and it had gotten really bad recently (waking up to completely dry, tight and flaky skin everywhere and having trouble sleeping). I have just gotten on methotrexate (one week into it) and have never used antihistamines. I’ve seen a lot of people talking about antihistamines helping out but I have no idea which one to get. Also if anyone knows are there any bad side effects that could go with it?