When you take a drug that slows down your immune system, like cyclosporine, methotrexate, or biologic drugs, your body can't fight off infection like it normally would. If you have an active serious infection, you shouldn't take these. You should also be tested to see if you've been exposed to tuberculosis or are carrying hepatitis B. Many antibiotics that treat infections shouldn't be taken with systemic drugs. Some change how well your body absorbs the psoriasis drug. Others keep your kidneys from flushing the medicine from your system. That can let it build up to dangerous levels.