Active efflux is a mechanism responsible for moving compounds, like neurotransmitters, toxic substances, and antibiotics, out of the cell; this is considered to be a vital part of xenobiotic metabolism. This mechanism is important in medicine as it can contribute to bacterial antibiotic resistance.

Efflux systems function via an energy-dependent mechanism (active transport) to pump out unwanted toxic substances through specific efflux pumps. Some efflux systems are drug-specific, whereas others may accommodate multiple drugs with small multidrug resistance (SMR) transporters.