Dosage calculations & extract equivalency in herbal medicine

Compound Tincture of RhubarbOver a number of years of teaching, one of the common issues I’ve noticed is that students often find calculating dosage equivalents rather challenging. It is important to be able to convert between liquid extract quantities and dry herb amounts, and vice versa. So I thought this would be a good place to discuss the issues and provide some techniques to help.

Extraction vs Dry Herb

For hundreds of years extracts have been used in the administration of herbal medicines. In the earliest and simplest forms this usually involved infusions and decoctions, however as tools and methods have developed over the centuries, far more complicated and sophisticated methods are available and commonly used today.

However for lack of a better measure, the most commonly used dosage standard is weight of dry herb material. This is used in most historical and modern texts, where you will see dosage statements such as …

2-4g of dry root or equivalent, three times daily.

The interesting and most important phrase in here is “or equivalent”.

Equivalency

Equivalency implies that another preparation form or extract of a herb is suitable, provided it is deemed to be “equivalent” to this dosage of dry plant material.

By definition an extract removes something and leaves behind something else. Extraction thus changes the chemical profile when compared to the dry herb itself. Many factors can determine the chemistry of the final extract including:

  • solvent (menstruum) – eg. polar or non-polar, or combinations of different solvents
  • duration of maceration of dry material
  • physical size of dry material being macerated
  • method used to extract (eg. percolation, or simple maceration only)
  • concentration steps employed (eg. none, heat, low pressure)
  • and of course the quality of the raw material being extracted.

Therefore in the truest sense an extract can never be completely “equivalent” to the dry plant material, regardless of how it is produced. Nevertheless this concept of equivalency is still used widely (indeed it is included as a concept in the Australian Therapeutics Goods Regulations) due largely to the common desire to employ mostly Galenical extracts.

Galenical Extracts & Selective Extracts

A general principle of herbal medicine is that the activity of the medicine is due to a combination of active chemicals in the plant or the extract. Thus in most cases it is desirable to create an extract which most closely represents the chemical complexity of the dry plant material. Such extracts are often termed “Galenical extracts”, and more recently “full-spectrum extracts”.

However extracts which are not really comparable to the chemistry of dry herb are sometimes used in herbal medicine, in particular in tableting. These extracts are often called “Selective extracts” as their production methods have selectively removed one or more chemicals to a high degree, at the expense of other chemicals. Sometimes so-called “standardised extracts” are in reality selective extracts, as the Australian regulatory definition for standardisation is somewhat loose.

The merits of Galenical and Selective extracts are debated at length in herbal medicine, and are beyond the scope of this document. Suffice to say that general or blanket statements on this issue are usually flawed, and choice of extract can be very dependant upon the desired outcome of the medicine. For example, a selective standardised extract of Silybum marianum has been demonstrated to have excellent efficacy in severe hepatic damage, and also very good safety, making it a useful clinical choice. However in many other cases selective extracts may offer no additional – or even reduced – therapeutic benefit.

Extraction Strength (Drug Extract Ratio)

The strength of herbal extracts are generally expressed as a ratio of dry plant material to final extract quantity, also known as the drug extract ratio (DER). It is expressed in the following format:

X : Y

X = weight of dry plant material

Y = weight or volume of extract (depending upon whether extract is a solid/dry extract, or a liquid extract)

The concept here is that the essential activity of the dry plant material (X) is found in the quantity of extract (Y). Or in other words, Y quantity of the extract is equivalent to X quantity of the dry plant.

Remember that ratios are fractions – ie. a 1:2 is the same as ½, a 1:3 is the same as 1/3 and a concentrated 4:1 is equivalent to 4.

DER Dry Herb Weight Extract Weight or Volume Extract Description
1:1 1kg 1L liquid extract (also by convention 1:1’s are known as a fluid extracts)
1:2 1kg 2L liquid extract
1:3 1kg 3L liquid extract
1:5 1kg 5L liquid extract; 1:5’s and below are often termed tinctures, although this is technically more determined by method of extraction than actual DER (i.e. technically tinctures are produced only through maceration)
1:10 1kg 10L liquid extract; tincture
2:1 2kg 1L concentrated liquid extract
4:1 4kg 1kg semi-solid extract – hence the change to using weight as a measurement of extract quantity rather than volume
8:1 8kg 1kg solid or semi-solid extract (depending upon consistency)
25:1 25kg 1kg solid extract
8-9:1 8-9kg 1kg solid or semi-solid extract; when extracts from different batches are combined to obtain specific outcomes (eg. standardisation or quantification of specific phytochemicals in the final product), sometimes a range is used as the extract ratio; this can make dosage calculations based on dry herb quantities difficult

 

Extract Equivalency & Dosage Calculations

With multiple dosage forms now being common in herbal medicine, it is essential that clinicians can convert between extract and dry herb equivalency, and vice-versa. This process is very simple, and it is essential that all herbal students and clinicians can do these conversions quickly and accurately.

For instance, need to be able to interconvert between …

  • an Echinacea tablet stated to contain 200mg of a 4:1 extract, with a dose of 1 tablet three times daily
  • an Echinacea purpurea radix liquid extract stated to be a 1:2
  • a textbook recommended dose for Echinacea purpurea radix of 1-2g three times daily

So let’s look first at some rules to help you make these conversions.

1) Converting from dry herb to extract (DIVIDE)

An example of this would be:

“You have a 1:3 extract and need to give 1.5g of dry herb equivalent. What dose of the 1:3 extract should you use?”

X (dry herb quantity) = 1.5g
Y (extract quantity) = unknown
Extract ratio: 1:3 (or 1/3)

Rule: Divide the dry weight by the extract ratio

1.5g divided by 1/3 = 4.5mL

Therefore the answer is 4.5mL

2) Converting from extract to dry herb (MULTIPLY)

An example of this would be:

“You have 2mL of 1:3 extract, what is the equivalent quantity of dry herb?”

X (dry herb quantity) = unknown
Y (extract quantity) = 2mL
Extract ratio: 1:3 (or 1/3)

Rule: Multiply the extract quantity by the extract ratio

2mL multiplied by 1/3 = 0.66g

Therefore the answer is 0.66g

3) Don’t Get Your Units Messed Up!

It is essential that you keep your units correct throughout your calculations. When you look at the table given on page 2, you see that a 1:2 extract is said to be 1kg : 2L. This is the same as:

1000g : 2000mL (because there are 1000g in 1kg and 1000mL in 1L)
1g : 2mL
1000mg : 2mL
500mg : 1mL

Some Examples

Here are some examples of applying the methods discussed above.

1) What is the dry herb equivalent of 2mL of a 1:2 extract?

This is an “extract to dry herb” question, so we multiply by the extraction ratio.

2mL multiplied by 1/2 = 1g

Therefore the answer is 1g

2) What is the 1:5 liquid equivalent of 3g of dry herb?

This is a “dry herb to extract” question, so we divide by the extraction ratio.

3g divided by 1/5 = 15mL

Therefore the answer is 15mL

3) What is the dry herb equivalent of 150mg of an 8:1 extract?

This is an “extract to dry herb” question, so we multiply by the extraction ratio. But remember that we need to keep our units the same!

150mg multiplied by 8/1 = 1200mg

Therefore the answer is 1200mg (or 1.2g)

Copyright Ian Breakspear, 2014