I Am On Opdivo, But NOT For Lung Cancer Or

I am on opdivo, but NOT for lung cancer or melanoma, my doctor has me on a regimen of opdivo following a regimen of docirubicin (sp?) and sprectrazocin (sp?). my tumor has shrunk, but probably will never completely go away. i am having problems taking deep breaths, and my other symptoms seem to mimic the cold-like symptoms of post-nasal drip, runny nose etc for which my GP prescribed Singulair/montelukast 10mg. will it hurt to take singulair so that i can lay down and sleep? 4 years ago, Singulair worked wonders for these same symptoms but it was before i had cancer diagnosed...