Pharmacokinetics and pharmacodynamics: Introduction

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Chapter 14 Pharmacokinetics and pharmacodynamics

Introduction

A good grasp of the principles of pharmacokinetics and pharmacodynamics are vital for understanding the actions of orthodox drugs, herbs and supplements. Without such an understanding, you will at best give ineffectual remedies, at worst be responsible for a foreseeable adverse reaction.

Certainly, as orthodox medication becomes more diverse and complex in its actions, it is necessary to keep up to date with possible interactions. Studying the interactions of the chemical constituents of drugs, herbs and supplements with the body and each other should also include a good basic understanding of the physiology of the system involved. So no aspect of pharmacology should ever be studied in isolation.

The terms ‘pharmacodynamics’ and ‘pharmacokinetics’ are often used interchangeably. This is usually due to a lack of understanding of the definitions:

• Pharmacokinetics: studies what a body does to chemicals (in drugs, herbs, supplements).

• Pharmacodynamics: studies the effects of chemicals (drugs, herb or supplements) on the body.

Four basic processes are involved in pharmacokinetics:

1. Absorption: how and where the substance enters the body.

2. Distribution: where the substance goes when it is absorbed.

3. Metabolism: breakdown or detoxification of a drug.

4. Excretion: removal of the substance or its breakdown products from the body.

The chemical make-up of the drug or herbal remedy influences:

• Where and how they are absorbed.

• Where they are distributed to.

• Where and how they are metabolized.

• How quickly they are removed from the body.

For ease of reading and comprehension, pharmacokinetics has been divided into:

• Routes of administration: how you get drugs into the body.

• Distribution: how drugs get into cells.

• Metabolism: how the body deals with drugs.

• Excretion: how the body gets rid of drugs.

Pharmacodynamics is covered in Chapter 19. Usually, this section of pharmacology involves calculations. These have been omitted as they are not necessary for the scope of this book.

For the sake of simplicity the term ‘drug’ will also mean remedy or supplement unless another term is more appropriate.

Bibliography

Neal (2005) Neal MJ. Medical pharmacology at a glance, 5th edn. Oxford: Blackwell Science; 2005.

Rang HP, Dale MM, Ritter JM. Pharmacology, 6th edn. Edinburgh: Churchill Livingstone; 2007.

Waller DG, Renwick AG, Hillier K. Medical pharmacology and therapeutics, 2nd rev. edn. Edinburgh: Elsevier-Saunders; 2005.

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