What are the causes of sudden itching, all over? For a long period

Allergy. This needs workup of there is no visible rash other than swellning for external infection such as scabies or an allergy reaction to an underlying infection, drug, or food. Start with oral anthistamine-allegro or zyrtec (cetirizine) and Saran for relief. Keep a food diary amd see dermatologist for blood tests and possible skin biopsy.
Let me help you. The medical term for itching is pruritus. Itching reasons : these are usually temporary and asudden discharge of histamine due to any of the above reasons will trigger of a itching, eczema, foodallergy, dermatitis, psoriasis, urticaria.