you can tell them, just don't be descriptive
for example, heres what you should say,
"be right back, gonna go to the toilet"
heres what you should NOT say
"hold on, i'm gonna go to the toilet, sit down on the toilet seat with the stale piss touching my *** cheeks, dump a fat stinky log into the dirty toilet water, and walk out of there without wiping or washing my hands"
ofcourse that was an exaggeration, but you get the idea