Meadowsweet vs. poplar buds and salicylous tree barks.
Externally, all salicylate herbs are painkillers. That's the flowers and green parts of the scented Filipendula -species (which excludes dropwort (Filipendula vulgaris - no meadowsweet scent), poplar leaf buds (= balm of gilead buds) (= Populus balsamifera) and the inner (green) bark of willow (Salix), poplar (Populus), aspen (Populus tremula), birch (Betula) and a few others.
Externally again, the balm of gilead buds and the barks go deeper into the tissues than the flowers and flowerbuds, but you can get the same effect by adding heating herbs (like cayenne, or ginger) to your meadowsweet salve (or bath, or poultice, or whatnot).
And of course, barking fallen trees is a lot of work ... not that the sticky mess of resinous popler buds is easy to pick, in spring.
Internally, they're very different.
The barks are loaded with salicylates - and tannins. Those tannins are tannic or gallic acids.
The balm of gilead leaf buds are loaded with salicylates and resins.
The meadowsweet and other Filipendula flowers and flowerbuds with the characteristic scent are loaded with salicylates and mucilage, while their greens are loaded with salicylates and some tannins.
And aspirin is acetylsalicylic acid.
That means that I only use meadowsweet (and other Filipendula) flowers internally ... painkiller, sure, but what I want out of these herbs is their gut soothing action. I give them to people with stomach or duodenal ulcers. And sometimes as a herb to help sweat things out.
You can take any salicylate herb (or aspirin, or similar) for the aches and pains of the flu, but be aware that if you do so you're clobbering your immune system ... that can be dangerous, especially in the elderly and weak, and especially if the current flu is a bad one.
I'd never even contemplate giving the barks or the leaf buds to people with ulcers - they don't contain the soothing mucilage. Instead they contain various acids or resins, which makes them way too harsh for an already upset stomach mucous membrane.
It's as bad an idea as giving aspirin, which actually is known to cause ulcers.
Wintergreen (Gaultheria procumbens) - we don't have wintergreen and I've never played with it, so I can't say where it'd fit among the salicylate herbs.
Any I've missed?