Depends on WHAT KIND of “drain” your talking about? Soil,waste, storm, acid,industrial
For house hold use normally keep grease out as much as possible. Sanitary napkins are a NO NO.
Some folks wash/water their plants in the kitchen sink sorry BUT potting soil isnt what a kitchen drain is intended for. Rags, certain food particles like bones rice coffee grinds should also be avoided. Pampers, Q tips, beepers and combs should not be flushed down a toilet BUT a lot of folks do try to. Beer cans dont pass the bends either. Have a great one