Preparations

What it is

The forms in which plant material is prepared for use, and why the choice of form is a pharmacological decision rather than a stylistic one.

This article describes the categories. It gives no ratios, no timings, no quantities and no instructions, and that boundary is deliberate.

The main forms across traditions:

• Infusion — hot water poured over soft material and left to stand. Leaves and flowers.
• Decoction — hard material simmered in water. Roots, bark, seeds. The standard form in the Chinese tradition.
• Cold infusion — steeped without heat, where heat would destroy what you want.
• Tincture — extraction in alcohol, usually at a stated plant-to-liquid ratio and alcohol percentage. The dominant form in modern Western practice.
• Glycerite — extraction in glycerol, used where alcohol is unsuitable.
• Infused oil and salve — plant material steeped in a fixed oil, optionally thickened with wax.
• Essential oil — volatile fraction separated by steam distillation. Not a herbal preparation in the same sense at all, and hugely more concentrated.
• Powder, pill, capsule — dried material ground, with no extraction step.
• Standardised extract — processed to contain a stated percentage of a named constituent.

Where it came from

Water and heat are the oldest and were universal. Wine and vinegar extractions are classical. Distillation was developed in the Islamic world — the alembic and the word alcohol both come from Arabic — and was taken up in Europe from the twelfth century, which is where the tincture ultimately comes from.

Paracelsus argued in the sixteenth century that the point of preparation was to separate the active principle from inert plant matter. He was right, and the isolated compounds described under when herbalism became pharmacology are the end of the road he pointed down.

Standardised extracts are twentieth century, and are the pharmaceutical industry's method applied to plants.

How it is used

By the methods listed above, all of which are documented in every herbal ever printed. Nothing further is described here.

What we can and cannot say

We can say the solvent determines the chemistry, and that this is the whole point. Water extracts water-soluble compounds — tannins, mucilage, many glycosides. Alcohol extracts resins, alkaloids and volatile oils that water leaves behind. Oil extracts lipid-soluble constituents. A water preparation and an alcohol preparation of the same plant are different medicines, and comparing a trial of one against traditional use of the other is a category error that the evidence literature makes constantly.

We can point to the case where this decided a Nobel Prize. Tu Youyou's artemisinin work stalled because the standard hot decoction destroyed the compound. Ge Hong's fourth-century text specified soaking in cold water. Changing the extraction was the discovery. A preparation detail, taken seriously, was the entire thing.

We can say the same choice can decide whether something is toxic. The kava liver injury cases in Europe involved acetone and ethanol extracts, sometimes of aerial parts; traditional Pacific practice uses an aqueous preparation of the root and has a markedly better safety record. Whether that fully explains the difference is contested, but the principle is not: a stronger solvent pulls out things the traditional method left in the plant.

We can say concentration is the most misunderstood variable. An essential oil is not a stronger tea. Steam distillation isolates a fraction that may be present in the whole plant at a fraction of a per cent, and several essential oils are seriously toxic taken internally — wintergreen is methyl salicylate, and a small quantity is a substantial aspirin overdose. Green tea as a drink is uncontroversial; green tea extract is now a leading identified cause of herb-induced liver injury. Concentrating a plant is not intensifying a tradition, it is making a new product.

And we can say why standardisation is both the honest answer and a partial one. A standardised extract tells you how much of one named compound you are getting, which is the single biggest improvement available. It also assumes the named compound is the active one, which is frequently assumed and rarely established — and if a plant's effect depends on several constituents together, standardising to one may select for the wrong thing. Practitioners who prefer whole-plant preparations make exactly this argument, and it is a reasonable argument that has not been settled either way.

We cannot tell anyone how to prepare anything, and will not. What can be said is that the form is not a detail: it changes which molecules end up in the cup, and the traditions that specified it precisely were specifying something real.

Further reading

• Tu Youyou's Nobel lecture (2015), for the cold-water problem in her own words.
• Any pharmacognosy textbook, on solvent selection — the chemistry is not controversial.
• The LiverTox entries for green tea extract and kava, for what concentration does.