Hello, coumadin has nothing to do with fluid retention. Her basic problem, congestive heart failure (CHF), is the most likely cause. In cases where patients with CHF accumulate fluid under their skins (edema) the diuretic dose is increased depending on the blood pressure and electrolytes. Claritin is safe to use but whatever drug she takes, the INR has to be checked because Coumadin action is a little unpredictable at times. If she s having itchiness because of the drug - which signifies an allergic reaction - perhaps she should change to another option regarding her treatment. There are other drugs that may be used for atrial fibrillation (to make the blood thinner), both pills and subcutaneous injections. Sometimes (depending on the patient history) an aspirin can be enough. I hope I ve helped! You can contact me if you d like to ask for clarifications or further information. Kind Regards!