Reheating coffee in a plastic cup in the microwave can be potentially hazardous and is generally not recommended. The reason for this is that certain types of plastic cups may not be microwave-safe, and when exposed to high heat, they can melt or release harmful chemicals into the coffee.
If the plastic cup is labeled as microwave-safe and is made from a suitable material, it should be able to withstand the heat from reheating coffee without melting or releasing harmful substances. However, even if the plastic cup is microwave-safe, it can still get very hot during the reheating process, so it's essential to handle it carefully to avoid burns.
To be on the safe side, it is better to use a microwave-safe glass or ceramic container to reheat your coffee. These materials are less likely to pose health risks or cause unwanted effects. Always check the labels on plastic cups or containers to ensure they are safe for microwave use before reheating anything. When in doubt, opt for microwave-safe materials or use a stove or other heating methods to reheat your coffee.