Is this the image of a marijuana user's lungs? No, that's not true: It's half of a walnut kernel. Also, this same post was in found in 2018 on a satirical Facebook account.
The claim appeared in an August 5, 2021, Facebook post (archived here). It featured an image of half a walnut kernel. The caption read:
This is the lungs of a marijuana user. It's really sad what this dope can do to your body. Please people be smart, put down the dope and pick up the hope.
This is what the post looked like on Facebook on August 16, 2021:
(Source: Facebook screenshot taken on Mon Aug 16 20:01:06 2021 UTC)
A Google reverse image search revealed the origin of the claim came from the account of user "Robert Soap Sopolosky" who posted the exact image and caption on July 24, 2018.
A line underneath Robert Soap Sopolosky's name on the Facebook page reads:
For Entertainment Purposes Only
Search results of Sopolosky's page show multiple other humor/prank posts.
