Any plastic will leach/outgass some of the materials it was made from. The word plastic means that it's not a solid. Some of those chemicals are harmful and some are not. The canteen is made to carry water. Water is usually pretty inert and dissolves the plastic very little. Beer is about 5% alcohol and alcohol is a solvent. So you will get more of the chemicals from the plastic that you wood just drinking water. Since you really never know if the plastic it was made out of is totally safe I would not drink beer out of a new canteen. If I had one that had been used to carry water in then I wouldn't worry much about it, you've probably already drunk up all the bad chemicals in the plastic. Just be sure to clean it out really well before and after a drinking binge. You should do this with water also.