Green’s Dictionary of Slang

(UK black) a drug user.

2014		Grizzy ‘Mandem Salute’ 🎵 Nittys want the drugs they be feeling it.
2017		Eve. Standard 4 July 8/4: Their graphic vocabulary — skengs (guns), borras (knives), nittys (drug addicts), neeks (drug dealers).
2020		G. Krauze What They Was 17: There were always nittys [...] damp and sticky-skinned with rotten black mouths [...] and clothes smelling sour .