Neurons are brain cells that both transmit and receive chemical messages called neurotransmitters, that generate our thoughts feelings and actions.

One important neurotransmitter is noradrenaline; noradrenaline is the fight or flight chemical that prepares us to respond to threats. It increases heart rate and blood pressure, widens air passages in the lungs, dilates the pupils to improve vision and narrows blood vessels in organs that aren’t needed for defence, such as the stomach.

Another important neurotransmitter is dopamine, dopamine controls our movements, keeps our attention focused, and our memory working well. It’s also the key chemical responsible for those wonderful feelings of pleasure when we engage in everyday things that are necessary for our survival, such as eating, drinking, and sex. When we feel good we’re very likely to do these things again and again. 

Now, along comes meth. Meth works by forcing neurons to release noradrenaline and most of their stored dopamine all at once. In fact, meth raises dopamine levels by more than ten times that of any pleasurable activity. In this way, meth causes people to feel energetic, awake, and alert, with an intense rush of pleasure or euphoria that’s unequalled by any natural activity. This powerful euphoric effect is a major reason why people can become dependent on meth so quickly.

Meth also stops the transmitting neurons from recycling excess dopamine, which is why the pleasurable feelings stay around for such a long time. The problem is that neurons can only store a certain amount of dopamine at any one time. 

Think of a glass full of pleasure messages and after a short period of overstimulation by meth, the glass quickly empties. Even if people keep using meth they still won’t get the rush they are after, because there is just not enough dopamine left to tell the brain to feel good. It’s as if your brain has run out of petrol and no matter how many times you pump the accelerator, you just can’t get going. 

It can take from two to ten days to restock dopamine stores, and during this time people can feel pretty flat, moody, irritable, forgetful, restless and exhausted. Kind of the opposite of using meth, it takes rest, a good diet, healthy exercise and most of all time to recover. 

This is also when people often get strong cravings to use meth again. If people continue to use meth regularly, the brain can’t cope with so much stimulation, and over a time it shuts down many of its dopamine transmitters and receivers. 

People then find that they have to use more and more meth, in higher and higher amounts to get any sort of rush. This is called tolerance. When meth tolerant people stop using meth, it can take months or even a year for the brain to fully recover. This is why people tend to feel depressed, moody, or irritable until dopamine can begin to work normally again.