The only side effect most of these products have is odor. Some of them were reviewed as being neutral to actually smelling good, while others were reported to have quite a strong smell you might not like. A lot of this has to do with the nature of the ingredients in pain relieving treatments. Mentholated ingredients give natural relief, but they do tend to have a strong odor that accompanies them.
Traditional wisdom says that NSAID pain relievers only damage your gut lining if you take them every day for a long time, but recent research disagrees. High-level athletes with stress-related intestinal damage tried taking ibuprofen to improve muscle soreness and recovery. Ibuprofen ended up damaging their gut lining even further after just a couple weeks; it increased inflammation and made their original pain issues worse. In fact, a single dose of aspirin can significantly increase your intestinal permeability.