I make my own juices, not putting different vendors juice together. Now I do use the microwave at 5 seconds for 15 ml bottle before I add the nicotine. I never have had any problem. If I make a smaller bottle I only put it in for 3 seconds. Shaking it several times while it cools. It is only warm not hot.. After it has cooled I add the nicotine. I have been doing this for a year. I wouldn't recommend it if you are combining juices from vendors as they already have the nicotine added..