The cups are made of paper, which means it is not un-recyclable. However, people are discouraged from using the same cup more than once because the paper may have traces of their saliva left on it which can cause illness. Another reason why people shouldn't reuse cups out of hygiene concerns is that bacteria can grow in hot liquids like coffee and tea. This would make it hazardous to re-use any warm liquid container, not just from Starbucks.