How our Bodies use Medication
Clinicians have spent decades studying what happens to drugs once people have taken them. This area of study - pharmacokinetics - analyses what the body does to a drug: for instance, where the drug goes and how fast, the way it is processed, and how it is cleared from the body. Pharmacokinetic studies focus on four processes:
• absorption
• distribution
• bioavailability
• excretion
It is these four processes that Phase I clinical trials assess. The data from these trials tell the physician how much of the drug to give and how often.
Absorption
Every medication has its own special chemical properties that affect how quickly and how completely it is absorbed into your body. Some drugs are broken down in the stomach and are not absorbed at all, while others cannot enter the bloodstream from the intestine and pass through the digestive system.
Drugs are classified according to their properties, for example:
• fat soluble – they dissolve easily in fat tissue (e.g. vitamins A and E, general anaesthetics, benzodiazepines)
• acidic – much like the stomach’s environment (e.g. aspirin)
• proteinaceous – a protein (e.g. insulin, the interferon beta class of drugs)
• size
Distribution
Once absorbed by your body, the drug spreads throughout the body in blood vessels. Blood vessels form a vast highway system that penetrates every area of the body. They provide a route by which the drug travels from one area (e.g. the point of entry) to another area (e.g. the target tissue – where the drug exerts its desired effects).
Bioavailability
Not all the medication you take is available for the body to use. The body processes or metabolises drugs to varying degrees depending on the point of entry of the drug. For example, 75-95% of some drugs taken orally are broken down in the liver before they reach the general circulation. This is known as the ‘first-pass effect’. Doses of these oral drugs have to be high to compensate for the amount lost in the liver.
Excretion
Once a drug has done its job (i.e. exerted its effects on the target tissue), it is cleared from the body via several organs, including the kidney (in urine), the gut (in faeces) and the lungs, and through sweat.
