What is the best treatment for drug overdose?

Depends... It depends on what agent has been overdosed. Some patients will require observation but others will require ICU level care, perhaps even life support! some overdosages are more dangerous than others do all overdoses need to be evaluated by a doctor to determine the care that is necessary.
Emergency Room. A drug overdose is a medical emergency. Initial management involves maintaining airway, breathing, & circulation. Eventually, if the person survives, rehabilitation & recovery are the answer.