Most of the time, OTC medicines help make us feel better and improve our health. But even OTC medicines can cause unwanted side effects. Most side effects are mild, but some can be life-threatening, particularly when large doses are taken or used. OTC medicines can also interact with other medicines you take, causing one or both of the medicines to not work as expected or produce unwanted side effects. Drug allergies are another type of unwanted reaction, which are particularly problematic in OTC combination products that contain aspirin. About 10% of the population is allergic to aspirin and other non-steroidal anti-inflammatory medicines. In this section, we cover frequently asked questions about drug interactions as well as adverse reactions that have sometimes been fatal in babies who have been given benzocaine-containing products for teething pain, or in adults who have applied too much OTC topical medicine.