The special value of locally-sourced powdered willow bark in relieving agues and inflamed, painful joints was reported to the Royal Society in 1763 by the Reverend Edward Stone of Chipping Norton, who suspected it should be just as useful as quinine from the bark of the Cinchona tree, Peruvian bark, because they were both bitter. He made a small test in about 50 patients that demonstrated both the efficacy of the preparation made from Salix alba and suggested an appropriate dose. There was subsequently growing acceptance and use of this treatment. Many developments in chemistry were required before the quite different active substances in the two types of bark were identified. Between 1828 and 1838 in France Leroux and Piria isolated the active principle from willow bark, which was called Salicin, after the tree, and showed it to be a compound of a sugar and salicylic acid. The chemically very different quinine in Cinchona was isolated in 1817 by Pelletier and Caventou and its structure was proven by Rabe in 1907. The history of the identification of the specific therapeutic role of salicylic acid itself and its evolution into Aspirin has partly been obscured by Nazi antisemitism in the 1930s-1940s. Following Stone’s demonstration of the therapeutic value of an extract of powdered willow bark salicylic acid was identified as the active component in the 1870s. Using that compound as a medicine proved difficult because irritated the stomach and produced such abdominal pain vomiting and diarrhoea that most patients refused to take it. About that time Bayer, a successful German chemical manufacturer, decided to invest in research including pharmaceuticals. In 1896 Arthur Eichengrȕner was appointed to lead research in a new laboratory working with another pharmacist Heinrich Dresser and a chemist Felix Hoffmann. The latter was looking for a better treatment for rheumatism from which his father suffered. In 1897 Hofmann synthesised derivatives of salicylic acid in which the irritant acidic group was masked by combination with simple alcohols. Use of ethanol produced acetylsalicylic acid. Initially Dresser considered it toxic to the heart but that was soon disproven in laboratory tests and when Eichengrȕner himself safely took it. Subsequent pragmatic trials in patients showed that that compound was a powerful anti-inflammatory and analgesic medicine well tolerated by most people. Before marketing the drug worldwide a convenient tradename was required and Aspirin was invented as a combination of ‘acetyl’ and ‘spirin’ from the source of the salicylic acid used - the meadowsweet plant, then called Spirea ulmaria. Aspirin has been a very successful medicine since then with many thousands of tonnes being taken each year for many painful and inflammatory conditions. Its use has extended into the prevention of certain blood clotting disorders and it shows promise in preventing certain types of cancer and possibly other disorders. Like all medicines it does have adverse effects; in this instance a particular risk of stomach haemorrhages. It took until 1971 for its mechanism of action to be proven when John Vane showed that Aspirin inhibited the synthesis of powerful, inflammatory chemical messengers known as prostaglandins. The same process underlies the risk of intestinal damage and haemorrhage.

It is always a bit odd that the bark of Salix which contains salicylic acid used since the time of Hippocrates to the present day for burning off warts and corns could be taken orally. Edward Stone's claims in Philosophical Transactions 53:195 et seq. (1763) that it was as good as Cinchona bark in curing intermittent fevers (malaria) have been taken uncritically by modern historians, but his contemporaries were less trusting. The Swede, Peter Bergius, said he had tried it several times for intermittent fevers (ie malaria) and it did not work - 'an undertaking with no effect'. See also William Woodville 'Medical Botany' (1793) p. 543. If it was the salicylate/aspirin like effect, why are there no trials of aspirin in malaria? (note by Henry Oakeley, 2019)

There is further information about the Reverend Stones' first use of a preparation of the bark and the subsequent history of the development of Aspirin in a recent paper (Wood, 2015).

It is the White Willow, Salix alba, that is historically the more relevant species. Salix alba is licenced today as a Traditional Herbal Medicine for the relief of backache, rheumatic pain and general aches and pains in the muscles and joints. Records of the medicinal uses of local species of Willow extend over many countries and many centuries. As long ago as 4000-1300BCE the Assyrians, Sumerians and Egyptians drank infusions of willow leaves to relieve painful musculoskeletal and joint disorders and to treat fevers. Perhaps surprisingly neither use is mentioned in the classical works on plants and medicines in Greece and Rome or in the later works on Materia Medica in Arabia or Europe in the Middle Ages. However, In the 17th-18th centuries infusions of native willow bark, leaves and even the seeds had several accepted uses in British medical practice; for example Culpeper’s 'A Physicall Directory', his English translation in 1649 of the College’s Pharmacopoeia Londinensis (1618) recommended them for treating fevers; ‘the boughs stuck about a chamber wonderfully cool the air and refresh such as have fevers’, and stopped spitting of blood, and the leaves applied to the head ‘help hot diseases there and frenzies.’ The idea that the willow branches in the sickroom would cure fevers had appeared with almost the same wording, nine years earlier in John Parkinson’s Theatrum Botanicum of 1640, and (in Latin) in Mattioli’s De Epitome Plantis of 1536. None of the 16th or 17thC herbals mention oral willow extracts for treating fevers. In this century, the only use for salicylic acid, the main medicinal compound formerly derived from willow bark, remains the topical treatment of warts and corns, as recommended by Hippocrates. Like Aspirin it is now manufactured by industrial chemical synthesis. (Summary by Henry Oakeley, 2018)

There is further information about safety review of Willow Bark from the United States Pharmacopeia Safety Review of Willow Bark.

The bark of Salix purpurea is anodyne, anti-inflammatory, antiperiodic, antiseptic, astringent, diaphoretic, diuretic, febrifuge, hypnotic, sedative and tonic. It is a very rich source of salicin, which was the original source of aspirin. The bark of this species is used interchangeably with Salix alba. It is taken internally in the treatment of rheumatism, arthritis, gout, inflammatory stages of auto-immune diseases, diarrhoea, dysentery, feverish illnesses, neuralgia and headache.

The flexibility and strength of Willow branches, which gives them that elegant appearance has been celebrated in poetry and paintings. They have led to many practical uses of different species in building, making furniture, in basket work, cricket bats, fishing rods; as biomass due to their rapid growth; as the source of charcoal for smelting and to stabilise wetlands. Willows have been important in a number of religions since at least 6000 BCE and they still retain a key role in Buddhist, Druidic and Jewish ceremonies as well as replacing the eponymous tree carried on Palm Sunday before Easter. In Asiatic countries willows are involved in ceremonies related to death and the passage of spirits between worlds. The Celts regarded them as possessing magical powers associated with the moon and water and in English folklore they had a dual reputation - a sinister one as they were believed to stalk unwary travelers, and a benevolent power to grant wishes made whilst knotting a slender branch.

In the countryside in England, pussy willows were eagerly sought by children for the silky catkin-bearing twigs called ‘palms’ to be used for church decoration on Palm Sunday.

Gastrointestinal bleeding and kidney damage possible. Avoid concurrent administration with other aspirin-like drugs. Avoid during pregnancy. Drug interactions associated with salicylates applicable.

The safety of Willow Baark preparations as food additives and as oral medicines is reviewed by the USP in Oketch-Rabah H et al (2019). United States Pharmacopoiea Safety Review of Willow Bark. Planta Med, 85, 1192-1205.