Bicarb of soda or a load of soluble aspirin. Leave overnight then flush.
Vodka is also apparently a miraculous household cleaner with a plethora of uses.
One shot (depends on the type of toilet though - not with a saniflo!) apparently will get rid of everything. It will smoke when it goes into water (wear gloves, don't inhale etc) and leave as per instructions. If it doesn't all go, repeat as necessary.