I am on prednisolone, hydroxycloroquine and an nsaid. Can I take antihistamine at the same time?

Maybe but it depends. I can't answer your question exactly b/c you didn't specify NSAID or antihistamine. Check out both www.Online.Epocrates.Com & www.Drugs.Com to check for medication interactions. Plug in what you're taking and both websites will warn you of possible interactions. They're not 100% foolproof but they're a good start. Better yet, ask whoever prescribed your meds. Always tell ur doc what you're taking!