All human emotions and passions, including the sensation or feeling of euphoria, are a reflection of the concentrations and movement of certain chemical substances in the brain. These substances as a group are called neurotransmitters.
Neurotransmitters are the real messengers of thinking and also permit communication between nerve cells (neurons). Together with other body mechanisms, these neurotransmitters direct such functions as appetite, attention and blood pressure.
Almost all drugs that affect the brain work by changing the level or activity of these neurotransmitters. Cocaine in the brain changes the concentrations and movement of neurotransmitters in a substantial way.
As the brain neurotransmitters are changed, so are the person's feelings changed. Cocaine can lead to feelings of euphoria. Sometimes a "rebound" effect follows, perceived as the feeling of depression.
Here's how the whole process works biologically.
Euphoria (that sense of happiness and well-being everyone wants to have) is regulated by the same group of neurotransmitters (like Dopamine) which regulate appetite, attention and blood pressure. When neurotransmitters send messages to the brain, stimulants (like cocaine) interfere with the normal communication process.
How does cocaine ("blow" in street slang) interfere with the body's normal function? Cocaine changes the flow (or travel patterns) of neurotransmitters by blocking their passage. The result disrupts the body's normal function and, initially, creates a highly stimulated condition followed, frequently, by a highly depressed state.
In this animation, we initially see normal body activity. When cocaine molecules are introduced, neurotransmitters (like Dopamine) cannot deactivate because the process of "dopamine reuptake" (or deactivation) is blocked. (Look at number 7 on this graphic.)
The result produces increased stimulation of the central nervous system, the heart muscle and the vascular smooth muscle. These changes can adversely affect a person's heart. (Heart problems are often the cause of cocaine-related deaths.)
Cocaine negatively impacts blood pressure and oxygen supply/demand. Because it is frequently inhaled, cocaine also causes lung and other pulmonary problems.
A person who habitually uses cocaine is virtually assured of developing some type of serious medical problem. Worse, those problems can occur on the first try. Even with a small dose.