Only if the container says microwave safe. Most foam takeout boxes are not rated for microwave heat and can warp or leak.
Foam containers look identical from one restaurant to the next, but the plastic underneath varies a lot, and only some of it is stabilized for microwave heat.
The risk is not dramatic failure so much as quiet failure: the container softens, sags, or leaches into the food before you notice anything is wrong.
A container marked microwave safe has been tested by the manufacturer to hold its shape under microwave heat, usually up to a specific temperature and time limit printed near the symbol. That certification does not carry over to other foam products, even ones that look identical from a different restaurant or store, so check each container rather than assuming based on past experience with a similar one.
Standard polystyrene foam starts to soften at roughly 165 to 175°F, well within the range food can reach in a microwave in under a minute. Fatty or sugary foods heat unevenly and can hit hot spots above 212°F, which is enough to warp, melt, or partially dissolve unrated foam. The result is often a container that sags at the corners or develops a slightly greasy, waxy feel where it touched the food.
When unrated foam softens or melts, small amounts of the plastic can transfer into hot or oily food, which is exactly what the microwave-safe testing is meant to rule out. It is rarely a large amount, but there is no reliable way to know how much without lab testing, so the label is the only real signal you have.
Move leftovers to a microwave-safe plate, bowl, or glass container before reheating rather than testing the original packaging. This takes fewer than thirty seconds and removes the guesswork entirely, especially for reheats that run two minutes or longer.
Reheating oily food in foam for just a few seconds, assuming that is safer, still risks softening at the contact points closest to the food. Stacking two foam containers to save a dish also traps heat against the plastic longer than a single container would, which raises the odds of warping even at lower power settings.
When there is no visible symbol and no packaging left to check, treat the container as not microwave safe. Transferring food takes less effort than dealing with a warped container or wondering afterward whether anything leached into a hot meal, and most kitchens already have a plate or bowl within reach.
Check for the microwave-safe symbol before every use, not just the first time. When in doubt, the thirty seconds it takes to switch containers is cheaper than a ruined lunch.
Only if it is labeled microwave safe. Unrated foam cups can soften at the seams well before the coffee itself reaches a hot drinking temperature.
It means the plastic was not rated for that heat level, and small amounts of it may have transferred into the food. Discard that container going forward.
No. Some containers use microwave-stabilized foam and others use standard polystyrene, and there is no way to tell by looks alone, only by the printed label.
Avoid it. Those are packaging materials, not food-contact containers rated for heat, so move food to a proper dish first.