When do I need to take and oral antihistamine for itching as opposed to a topical cream?

Itchy Skin. When the area of itch is too significant for a small amount of cream to cover. And there is nothing wrong with doing both. Itching can be an internal issue or an external issue. Antihistamines help to drastically reduce itch and are longer lasting, depending on the topical cream/ointment used.
If cream not working. The time to switch from a cream to an oral antihistamine is if the itching/rash is widespread enough that you're needing to use a lot of cream or if the cream isn't working. You can certainly combine the cream and the pills. If you develop the symptoms during pregnancy or they don't get better with treatment, talk to your doctor. Rashes during pregnancy can be a sign of complications.