Lubricant (aka "lube") can be added to the outside and inside of a condom. Petroleum-based lubricants, like Vaseline, baby oil or edible oil, can cause latex condoms to weaken and/or tear. Instead, using water-based lubricants such as K-Y brand jelly ensures better usage.
Some people are allergic to nonoxynol-9, a lubricant and spermicide found in some male and female condoms.