Concept:
Cytotoxic drugs are hazardous drugs used mainly in cancer therapy. They can be harmful to humans and the environment.
Step 1: Hazard of cytotoxic drugs.
Expired cytotoxic drugs cannot be discarded like ordinary waste.
Step 2: Proper disposal.
They should be destroyed by high-temperature incineration to prevent exposure and contamination.
Step 3: Why other options are wrong.
Flushing in drain can contaminate water.
Municipal garbage can expose people and animals.
Recycling is unsafe for cytotoxic drugs.
Step 4: Final answer.
Thus, expired cytotoxic drugs are disposed of by incineration.
\[
\boxed{\text{Incineration}}
\]