When someone takes a drug for the first time, they experience its full effect. But if they keep taking it, their body adjusts. It takes more of the drug to produce the same effect.
This is known as tolerance. With opioids, tolerance can build after just two to three doses.
Tolerance happens through changes at the cellular level. It’s the result of the brain restoring balance after it’s repeatedly overstimulated by a drug. As balance is restored, neurons, synapses, and entire brain regions start to work differently. The brain sets a new normal that includes effects of the drug.