Why does a person get tested for illegal drugs after an overdose on legal medication?

Toxicology. If you overdose on a legal medication you are going to present with symptoms that may resemble an overdose of illegal medications, so part of the initial workup is to test for illegal substances. The best way to treat an overdose is knowing exactly what has been ingested. It is also assumed if you are capable of overdosing legal medications, then you may also be at risk of illegal drug abuse.