“Krokodil” is the street name for the semi-synthetic opioid derivative desomorphine.
It's commonly claimed that Krokodil will make your skin fall off:
https://www.drugs.com/illicit/krokodil.html
Those who inject these caustic agents into their veins can develop extreme skin ulcerations, infections, and gangrene -- a discolored (green, grey, black) scale-like skin
https://time.com/3398086/the-worlds-deadliest-drug-inside-a-krokodil-cookhouse/
Wherever on the body a user injects the drug, blood vessels burst and surrounding tissue dies, sometimes falling off the bone in chunks.
https://www.thesun.co.uk/news/2682111/pics-addicts-scaly-wounds-crocodile-injecting-drug-krokodil/
The drug turns a user's skin green and scaly around the area where they inject it as blood vessels burst and the skin rots away.
If properly prepared, the toxicology of desomorphine is similar to other opiates. However, contaminants which can be mixed in with the desomorphine due to lazy preparation can cause necrosis. Similar chemicals are used during mass production of heroin, cocaine, and methamphetamine, but they are more thoroughly removed before final packaging.
The claim that Krokodil makes people's skin fall off is no more accurate than a claim that heroin gives people HIV.
Source: https://sci-hub.se/https://pubmed.ncbi.nlm.nih.gov/24650492
Repeated administration of desomorphine can cause severe medical complications which include physical and psychological dependency, tolerance and a withdrawal syndrome if the substance is no longer taken, similarly to heroin (Grund et al., 2013; Erowid, 2013). The dependence potential of desomorphine was initially described by Eddy et al. (1957) and was verified later by Sargent and May (1970) during their study with monkeys. No further studies have been conducted after those concerning the dependence potential or its dependence liability of desomorphine compared to morphine. Other effects of desomorphine are similar to those of opiates, including miosis, flushing and paresthesia (neutral effects). Common negative effects are constipation and urinary retention, nausea and vomiting, as well as more serious medical implications, such as allergic reactions, seizures, and respiratory depression leading to death (Grund et al., 2013; Erowid, 2013). All the above effects are expected as they are inherent of an opioid like desomorphine.
The final product [of common street synthesis] is often contaminated with various toxic and corrosive by-products or residuals. Thus, synthetic analogues of codeine, other drugs such as tropikamide, ephedrine or acetaminophen (often found in codeine containing preparations), phosphorous, iodine or even heavy metals like lead can be found in the “krokodil” street samples, as a result of poor synthesis. All these substances are considered to be responsible for the most of the undesirable or toxic effects that appear after the repeated injections of “krokodil” which actually is a cocktail of all the above substances; desomorphine, by-products and residuals.