Nicotine gum, are there any side effects?

A few: the nicotine can cause stomach upset, heartburn and reflux esophagitis, especially in higher doses, and very high doses of nicotine can cause nausea, vomiting and worse, but this rarely, if ever happens with the gum, lozenges or patches. The gum is very thick and hard to chew, and can cause TMJ (jaw) pain, pull out loose fillings and even teeth, so that lozenges are my preferred recommendation.