Herbal teas and infusions: how to make them and how to judge them

An infusion is hot water poured over a plant and left to steep, the way most tea is made. It suits soft parts: leaves and flowers such as peppermint, chamomile, lemon balm and hibiscus. A decoction is a simmer: the plant goes into water on the heat for some minutes. It suits hard parts such as roots, barks, seeds and dried berries, including ginger root, cinnamon bark, fennel seed and licorice root.

Crushing fennel seed or grating ginger exposes more surface to the water, which is why a pinch of whole seed in hot water tastes much weaker than the same pinch lightly bruised.

Most herb trials use capsules, standardized extracts or oils, because they can be dosed and matched with a placebo. A cup of tea delivers a smaller and more variable amount. Peppermint is the clearest example: the evidence for gut symptoms is for enteric-coated peppermint oil capsules, and NCCIH discusses peppermint tea separately, noting it appears to be safe while the long-term safety of large amounts of the leaf is unknownNCCIH.

So read our grades as applying to the form studied. A tea can still be a pleasant, low-risk way to try a herb, but it is not evidence that the tea does what the capsule did.

Comfrey. Comfrey leaves and roots contain pyrrolizidine alkaloids, which damage the small veins of the liver (veno-occlusive disease) and can progress to cirrhosis and liver failure. Germany and Canada banned its distribution Stickel 2000, and liver damage has been reported from comfrey herb tea itself Ridker 1989.

Contamination in ordinary teas. Pyrrolizidine alkaloids also turn up as contaminants when weeds are harvested with the crop. A German study tested 44 herbal teas sold for infants and for pregnant or breastfeeding women and found these alkaloids in 38 of them, at widely varying levelsMadge 2015. Rotating between different teas, rather than drinking large amounts of one blend every day, limits exposure to any one contaminated batch.

Licorice root. It is safe as a flavoring, but regular strong licorice tea delivers glycyrrhizin, which can raise blood pressure and lower potassium NCCIH. See licorice and blood pressure medicines.

In the US, herbal products sold as dietary supplements are not approved by the FDA before sale. Their labels may not claim to diagnose, treat, cure, mitigate or prevent any disease, but they may carry structure or function claims, and words like “standardized,” “verified” or “certified” do not guarantee quality NCCIH. Look for:

• The Latin name of the plant and the part used (leaf, flower, root), so you know what is in the box.
• A single ingredient, if you want to know what you are reacting to; blends make that impossible.
• A lot number and best-by date, and a company you can contact.

If you want single-ingredient dried herbs that a local shop does not carry, iHerb stocks loose and bagged chamomile, peppermint, ginger and fennel. A supermarket spice aisle often has the same plant for less.

NCCIH notes that many supplements have not been tested in pregnant women, nursing mothers or childrenNCCIH. Peppermint in food amounts is likely safe in pregnancy, but little is known about medicinal amounts NCCIH. Chamomile can cause allergic reactions, more often in people allergic to ragweed, chrysanthemums, marigolds or daisies NCCIH.

Some teas interact with medicines, including chamomile with warfarin. Check the interactions list, and tell your pharmacist about any herbal tea you drink daily.