A patient describes a vivid, persistent sensation of insects crawling under the skin of both forearms, prompting frequent scratching, in the context of recent heavy stimulant use. No skin lesions or rash are found on examination. What is this?

This is a tactile hallucination, specifically the well-recognised pattern of formication (crawling under the skin), with no physical cause found on examination and a well-recognised association with stimulant use. It is not a genuine skin condition, which the normal examination rules out. It is not a visceral hallucination, since the sensation is located on/under the skin surface rather than inside the body's organs or deep tissue. And it is not, on its own, a delusion of parasitosis, since no fixed belief about actual insect infestation is described here — only the sensation itself, which, as the phenomenon's own text notes, can occur separately from any accompanying delusional belief.