A good rule of thumb is only flush the 3Ps: pee, poop, and toilet paper.
Never put fats, oils, and grease from cooking down the drain. Food scraps should be thrown in the garbage or composted, if possible. These things cause build up in the pipes that can result in sewer backups in your home. Defend your drains!
Never flush medications as this can contaminate local lakes, rivers, and streams.