To educate downvoters: Yes, “essential oil” health advice is a common bullshit indicator! But! They’re the standard pharmaceutical for this task, found in kits like this one used to train food and beverage scientists. Any diverse set of strong familiar smells can work.

I worked in a related field and got my ass kicked by long COVID shortly after the start of the pandemic. This is a good starter on the biology involved, a journal article from Nature, unpaywalled: https://web.archive.org/web/20220623072436/https://www.nature.com/articles/d41586-022-01628-9

Treating anosmia from brain damage is the same for post-COVID as for a car accident or getting gassed in WWI: you diligently breathe in the olfactory training kit and try to vividly remember those smells.

Pull quote: “Sometimes, the sense of smell recovers spontaneously after injury. The olfactory nerve is the only cranial nerve that can repair itself when damaged, and olfactory sensory neurons — cells in the upper part of the nose that recognize different odorant molecules — renew themselves periodically throughout life.“

edit to add, extreme sympathies to anyone who has found themselves experiencing this!

e2: corrected link