Whether a reusable plastic travel coffee mug can be microwaved depends on the type of plastic used in the mug. It is important to check the bottom of the mug to see if it is labeled as microwave-safe or check with the mug's manufacturer if there is no label.
Several different types of plastics are used for food storage and utensils, and not all of them are safe to microwave, according to the Harvard Medical School Family Health Guide. Some may partially melt or leach toxic chemicals into food and drinks when heated. Microwave-safe containers are usually made from either polyethylene terephthalate, high-density polyethylene or polypropylene. It is also important to check that the lid of the mug is made from a microwave-safe material.