Please don't think these two species are interchangeable! Lavandula stoechas can be toxic. Its very high in ketones and camphor and has a very different chemistry than L. angustifolia (syn. L. officinalis, syn. L. vera) (which is the variety that should be used therapeutically).
You can view the scanned pages from Geunther on Stoechas with the various constituents at my blog at https://sherwoodlavender.wordpress.com/. If you'd like to look at L. angustifolia let me know.
Ketones are easily absorbed by the skin and are quite irritating so we would never recommend L. stoechas for cosmetics. It is so important to distinguish the species and varietal of any botanical. I've seen some reports that lavender doesn't work as a sleep aid (because a study uses L. intermedia which is high in stimulating camphor and low in relaxing linalyl acetate) or that lavender causes irritation because a variety high in ketones or camphor is used.
Here's a case study on pubmed of someone who poisoned themselves with L. stoechas tea: http://www.ncbi.nlm.nih.gov/pubmed/22760025 and the full text is available here: http://www.kjm.keio.ac.jp/past/61/2/66.pdf
You'll notice the title of that case study: "Anticholinergic Syndrome and Supraventricular Tachycardia Caused by Lavender Tea Toxicity" would make the average reader thing that all lavender is similarly toxic - when its likely the fact that it was L. stoechas that caused the problem.
It is a great opportunity to remind ourselves that even if plants share common names, or are even the same species, they can still have very different chemistry and therefore their appropriate uses and toxicity can vary widely. Always be sure you know what the safe varieties are to use and that you have correctly identified your plant before you use it to make tea or any other product. If you're in doubt, simply enjoy looking at it in your garden...
Best Wishes,
Erika Yigzaw
Erika Yigzaw
ACHS CIO and Master Gardener
