I have a sore throat after coughing during a viral fever and can barely swallow my own saliva. Is a fexofenadine and montelukast combination tablet safe to take for this, and what's the dose and side effects?

That sounds really uncomfortable, and pain severe enough to make swallowing difficult is worth taking seriously rather than treating at home. This particular tablet is actually meant for allergic symptoms like sneezing and nasal congestion, not for throat pain or infection, so it may not touch what you're dealing with right now. Severe throat pain during a viral fever can sometimes mean a secondary infection that needs a different kind of treatment altogether, so please see a doctor soon rather than waiting it out - they can examine your throat and prescribe something that actually targets the problem.