"I have tried all sorts of medicines but have never tried marijuana for this."
Both internal and external cannabis should help with chronic psoriasis and both should be considered. Psoriasis is a systemic problem so it should be treated systemically. CBD cannabis should stop the inflammation internally and externally but the THC should stop the itching especially if you taken orally. There’s also a high incidence of vitamin D deficiency as well as zinc deficiency in most psoriatic sufferers. Keep working on high nutrition, reduce all inflammatory foods especially those containing omega 6 and correct nutritional deficiencies and your psoriasis should show marked improvement.