To understand how drugs work, we have to first have some understanding of how the human body is composed. We can do this by imagine we are slowly zooming into the body at its different levels of components:

• Body
  
  …zooming in…
  
• Systems e.g. the nervous system, the cardiovascular system, etc
  
  …zooming in further…
  
• Organs e.g. heart, brain, liver
  
  …zooming in even more…
  
• Cells
  …and finally, at the smallest level…
  
• All the stuff inside the cells e.g. your DNA, hormones and different chemical signallers

What happens in your whole body is dictated by what happens in each of those increasingly tiny, microscopic parts… right down to the DNA and chemical signaller level. We can imagine there are a bunch of chemicals acting within all of these parts – within the cell and between the cells, and therefore impacting the function of the organs, the systems in the body and your whole being.

There are a bunch of rules dictating how chemical signallers or messages get to where they should go and drugs are no different. Some key rules are:

1. Like attracts like
   Chemical signallers or messengers will move towards certain parts of the body if they are alike. There are some chemicals which will combine well and attract – like the way sugar and water mix together easily. There are others that don’t attract at all – such as the way oil and water don’t mix well. As a general rule, signallers and substances which are “fatty” will mix well with other “fatty” type things, whereas things which as more “watery” will mix well with other “watery” things.
   
2. There are boundaries
   The body has a bunch of different boundaries which allow substances to get to certain organs or body parts. What is interesting to consider is that if a substance can get into the blood stream, it can generally move fairly freely throughout the body. But the body also has a bunch of protective mechanisms to prevent potentially dangerous or foreign things from getting to certain parts of the body. One of the most interesting examples of this is what is called the “blood brain barrier”. This is a protective layer around the brain which prevents potentially dangerous substances from getting into the brain, whilst allowing other important things in and out.
   
3. Size matters
   In order to move through and around the body, it is important for a substance to be the right size. Some chemicals may move through the body by squeezing between two cells, others may be transported through particular pathways that move the chemical from one side of the cell to be spit out the other side. If a chemical is too big, it is unlikely to be able to move very effectively through the body.