[jack_ (OP)]

Mary has been in touch to ask:

How do pharmaceutical drugs target different areas of the body? For example, why do different antibiotics work better for fighting acne and some for fighting pneumonia?

What do you think?

[evan_au]

Sometimes it is in the way it is applied:
- If you put medication on your skin (topically), it will affect your skin
- If you swallow de-worming tablets, they will affect your gut (and not be absorbed into the blood supply)
- If you put drops in your eye, it will affect the cornea
- But if you inject it into the bloodstream, it will affect everything which has a blood supply (although it may have trouble crossing the blood-brain barrier)
- Similarly, if a small molecule pill is absorbed into the bloodstream, it can affect most of the body

There are other ways a medication can affect just part of the body, even if it is carried everywhere by the bloodstream:
- It may be targeted at a biochemical pathway that occurs in bacteria, but not in humans, so it affects that bacteria wherever it is exposed to chemicals in the bloodstream
-  It may be targeted at a biochemical pathway that occurs in humans, but which mostly occurs in one organ (eg the liver, or the thyroid)

Of course, biochemical pathways rarely exist exclusively in one organ, and so medications can often cause unintended side-effects in other parts of the body.
- Often, when a patient is admitted to hospital with multiple medications that have been prescribed over the years, the first step is to discontinue all medications for which there is not a clear and immediate need. Sometimes this improves the patient's condition significantly.