Review on Bottle Narrow Opening Adapter Leak Proof by Christopher Theriault

I love my Sigg water bottles but these plastic caps tend to get mold around the white plastic rim which no amount of bleach seems to prevent. Basically, you should schedule a replacement once or twice a year, depending on usage. If I removed the covers more often to let them dry, this might not happen, but not worth the hassle. You can also put the new covers in the washing machine before use, as they initially have a plastic smell (but this will disappear).