Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Losartan is usually safe unless you re allergic to it (the consequences depend on how serious this allergy is) or have renal artery stenosis (there is some risk for renal damage). Raising the blood pressure is not a side effect of either drug under normal conditions. In pheochromocytoma though atenolol may precipitate a hypertensive crisis if adequate alpha-blockage has not been achieved. Has your doctor checked for pheochromocytoma? Suspicious symptoms include headaches, postural hypotension, uncontrollable hypertension, anxiety, etc. Sometimes it s only due to anxiety. Having high blood pressure and having to change the regimen is stressful for most individuals. Some of them develop episodes of hypertension while the appear to be adequately controlled the rest of the time. So in conclusion adding losartan is normally safe. Checking for secondary hypertension is important in cases like yours. I hope you find my comments helpful! Kind Regards!