Biologically, there are not two types of dopamine. The molecule is the same, and the terms "fast dopamine" and "slow dopamine" are not scientific expressions, but rather popular terms used to explain different ways the brain works. What really changes is the way the reward system is stimulated: the speed, the intensity, the predictability, the effort required and the frequency with which the reward appears.
Dopamine is also not simply the "pleasure molecule." It participates mainly in motivation, learning and expectation, helping the brain identify what is worth repeating. When an experience produces a reward, especially if it is unexpected, we quickly learn to seek it again.
This is where what has come to be called "fast dopamine" comes in: these are nearly instantaneous rewards, obtained with minimal effort, such as opening a notification, receiving a like, watching a short video or moving on to the next content. These experiences stimulate the reward system intensely and immediately.
