Yes. And no. It depends, really.
You should never microwave anything that is not designated and labeled as "microwave safe," even if it holds up to melting. The FDA hands down "microwave safe" labels based not only on whether plastic holds its shape, but also whether or not (and how much) it leaches chemical components.
Notice we said "how much" it leaches. Some plastics are bestowed with the microwave safe label even if they leach chemicals, as long as it's within certain parameters (which depends on the type of plastic). So if you're seriously concerned about toxins, like phthalates and BPA, it would be best to eschew plastic in your microwave altogether.
If you enjoy the convenience of zapping a quick microwave dinner, you don't need to stick to an all-or-nothing rule. Instead, follow these precautions for microwaving plastic (from the Harvard Medical School Family Health Guide):
- "Safer" plastics are labeled with the numbers 2, 4 and 5. Plastic #1 is suspected to leach endocrine disruptors, depending on how it’s used and at what temperature. And Plastic #7 is the most likely to contain BPA. But beware: Not all plastics with the "safe" numbers numbers are safe for microwave use.
- Most takeout containers, water bottles, and plastic tubs, bottles and jars (like the ones that hold margarine or condiments in the grocery store) are not microwave-safe.
- Microwavable TV dinner trays and grocery plastic steam bags are formulated for one-time use only and will say so on the package.
- Before microwaving food, be sure to vent the container: Leave the lid ajar, or lift the edge of the cover.
- Don’t allow plastic wrap to touch food during microwaving. Even better, substitute plastic wrap with wax paper, kitchen parchment paper, or white paper towels instead.