Final answer:
An overdose on sedatives, barbiturates, or narcotics typically causes respiratory depression, which can lead to serious complications including coma or death.
Step-by-step explanation:
An overdose on sedatives, barbiturates, or narcotics is most likely to cause respiratory depression. These substances are central nervous system depressants and can lead to symptoms such as sluggishness, drowsiness, memory loss, and decrease in respiratory function. In severe cases, an overdose can result in coma or death due to the significant depression of respiratory function. When combined with similar CNS depressants, the risk of overdose increases significantly, due to their additive effects on the central nervous system and respiration.