
Willow bark has been used since at least the first century AD (Dioscoredes) for pain relief and to lower fever. Like many of the water loving anti-inflammatories, wilow bark contains salicylates, that are converted into salicylic acid in the gut. Salicylic acid is the basis of aspirin. Other species of willow (eg. crack willow, grey willow, black willow, bay willow, puple osier) can be used interchangeably with Salix alba. The bark, harvested in the Spring, preferably off branches that are two or more years old, should yield a red tincture or decoction, tasting quite bitter. The compounds you want are water soluble, so the tincture doesn't need to be any more than 25% alcohol.
Willow is used for any painful inflammatory conditions, and can be used both internally and externally. As part of a lotion or cream it makes a good back rub or liniment for muscle spasm and pain. Internally it is useful for fevers, colds, headaches, rheumatic conditions, gout, diarrhoea and dysentery.
The bark is the main part used for making medicines as it contains the highest concentration of salicylates, but the leaves can also be used when fresh and in season, for a perfectly satisfactory infusion for treating headaches, colds, fevers etc. You can dry the leaves to, but they lose a bit of their power over time, and don't keep too well.

