I'm on a septic system. Why does my toilet overflow from time to time has not rained. Vent pipe is cleared.

Several reasons... septic tank full and needs pumped (they do need pumped out from time to time to remove sludge)... drain pipe blocked for some reason.. (foreign object, collapsed pipe, ball of tree roots, etc).

Smoothy hit it on the head. On average septic should be cleaned out every 2 years. This depends on size of family.

Most companies that do this will send u a reminder when u are do for a cleaning.

Chuck

If it is only now and then, perhaps a slight blockage in a drain.
But if there is a clean out or port on the septic, you can also check to see if it is full