Systemic treatment -- treatment that affects the body's whole system -- is an approach that's typically used only in severe cases of psoriasis. This means psoriasis that hasn't been helped by other topical treatments or phototherapy or that covers more than 20% of the body. This type of treatment is usually reserved for those who have more severe types of psoriasis that don't respond to other treatments. While systemic treatment can help clear your psoriasis, many of the drugs can cause serious side effects. Your doctor will probably want to monitor you closely.

Here are some of the most common systemic treatments for psoriasis:

• Methotrexate (Rheumatrex and Amethopterin). Taken either orally or by injection, methotrexate suppresses your immune system, which in turn reduces the effects of psoriasis. Usually, you would take it once a week and see results after four to six weeks. Used in the short-term methotrexate can work very well. According to the National Psoriasis Foundation, this drug works by binding and stopping the steps involved in the rapid growth of cells.   
  
  Methotrexate can have serious side effects. In the short term, you might experience nausea, fatigue or insomnia. In the long-term, methotrexate can cause damage to the liver and blood cells. Make sure to tell your doctor about any other medical conditions you have, since methotrexate is not appropriate for people with diseases such as anemia, liver disease, or other ailments. You should also cut out alcohol, since even one glass a day in combination with methotrexate may be enough to cause liver problems. Women who are pregnant or thinking about becoming pregnant should not take methotrexate. Their male partners should also avoid this drug. While taking methotrexate, your doctor will do regular blood testing to monitor your blood cells and liver function. He or she may need to perform periodic liver biopsies, especially if you have a history of liver problems, hepatitis C or alcoholism.
• Cyclosporine (Neoral or Sandimmune). Another drug designed to suppress the immune system and slow growth of skin cells, cyclosporine is typically used in only severe cases of psoriasis where nothing else seems to work. It comes as an oral medication-in pill or liquid form. While it can help clear psoriasis, its benefits typically last only for as long as you're using it. Cyclosporine also carries risks. It can cause kidney problems, high blood pressure and high cholesterol. People who have weak immune systems or women who are breast-feeding shouldn't use it. People receiving PUVA therapy (a form of phototherapy) should also not use cyclosporine. Because of its toxicity, experts recommend that you not take the drug for more than a year at a time.
• Retinoids. Acitretin (Soriatane)and Accutane are drugs derived from vitamin A, and it works by affecting the way that skin cells grow and are shed. On its own, it works very well with pustular and erythrodermic psoriasis. For plaque psoriasis, it works best in combination with phototherapy. These medications can cause serious birth defects even after one has stopped taking it. Women who use these drug should not be pregnant or plan to become pregnant within three years after finishing treatment
• Hydrea (hydroxyurea). A medication designed to treat cancer, Hydrea has fewer side effects than some of the more potent systemic medications. However, it is also less effective. It does have some side effects of its own, such as problems in the bone marrow and an increased risk of skin cancer. Hydrea shouldn't be used by women who are pregnant or might become pregnant. While it has been used as a treatment for psoriasis for years, the FDA has not approved this use.
• Other drugs. A number of other drugs used for cancer, preventing transplant rejection and other conditions -- such as 6-thioguanine, Azulfidine (sulfasalazine), Prograf (tacrolimus), and CellCept (mycophenolate mofetil) -- are also used for treating psoriasis in some cases. While these drugs haven't been approved by the FDA for psoriasis treatment, they may still help.