Mercury may be toxic, but ammoniated mercury ointment was used to treat psoriasis in the not too distant past. It was as successful as the tars, without the staining.
If you are DESPERATE, then methotrexate will restore your hopes. Everyone responds to methotrexate. The rheumatologists have been using it for ALL forms of arthritis, rather freely. Years ago,when we first started using MTX (methotrexate) for psoriasis, it was noted taht the psoriatic arthritis responded sooner that the skin lesions. The side effects are mainly leukopenia and liver toxicity. The leukopenia is is usually preceded by sores in the mouth. Unfortunately the liver toxicity can only be seen with liver biopsies. It is most unlikely that you would have any problem with a few months only of MTX.
Dovonex is not initially effective on its own. It works best when used in conjunction with a high potency topical corticosteroid ointment(ointments work better than creams). After the lesions are about gone, the steroid is discontinued and the Dovonex applied twice daily.
MTX and other antimitotic drugs seem to work very well because the turn-over time of epithelial cells in psoriasis is sevenfold the normal rate. MTX reduce the rate of mitoses.
Bathing at the Dead Sea worked well, but might be more dangerous that MTX now. Larry