Get a Yellow Book with it's accompanying Pink ID Card and never have to worry about it again. (goes for your driver's license and a host of other things as well...).
Some of you may recall parts of the less than glorious history of Chaindrite when one death and several illnesses ended up killing a really nice hotel in Chiang Mai - The Downtown Inn. VERY careful with that...................
Available at pretty well all internet resellers. Have the Nano S. No issues - but suggest you use an official reseller (have heard of issues with grey goods).