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Didn't think they still prescribed that for P, there are far more effective treatments available nowadays.
I was on it about 10 years ago, first oral med I tried...didn't do a thing for me after 6 months on it apart from make my sweat smell, I then moved on to Acitretin along with PUVA treatment.
it was created 40 or so years ago for the treatment of Sickle cell anaemia, and as with a lot of P drugs accidently was found to affect the division of skin cells in P. It affects how DNA divides during cell mytosis. There was talk a few years back of a possible risk of leukeamia and bone marrow damage from prolonged use., not sure if anything came of investigations into that though.
If it works it is supposed to be effective after about 8 weeks of taking it from memory.