Any allergic reaction or symptom that is caused by a medication. The reactions can be mild such as nausea. They can be life threatening such anaphylaxis: low blood pressure, fainting and collapse. Small children and the elderly are most susceptible to medication reactions. A medication reaction should be considered as a possible cause for a sudden change in behavior or new symptom in children and the elderly. Over the counter medications can also cause serious side effects. Even medications that initially were tolerated without a problem can result in serious reactions. Some medication reactions are caused by taking too much of the drug requiring a decrease in a patient's dose. For specific reactions review the adverse reaction section of the medication being taken.