What is the safest systemic treatment for psoriasis that can be taken long term?

Many: All therapies have side effects. Long-term therapies include Methotrexate tnf inhibitors which can be taken over the long term. Safety is relative because each has side effects.

It depends: The major goal is adequate control of the skin involvement. The safest meds are the topical creams but these are really only effective for localized disease. Phototherapy is also fairly safe. Methotrexate has some greater risk but can be very effective if other agents are not. Finally, tnf agent carry greater risk but are extremely effective for moderate to severe disease.