Coronary slow flow phenomenon (CSFP) is present if it takes more than three heart beats for contrast media, when injected down a coronary artery, to pass from catheter tip to distal vessel. This is under non-emergent conditions in the cardiac catheter laboratory, and it is visually dramatic in an otherwise asymptomatic patient under study. It makes intuitive sense to attribute this to a restriction of blood flow into the myocardial microvasculature, therefore delaying dye transit, because of coronary microvascular dysfunction (CMD). However, there is a clear distinction between contrast and blood clearance; while the former is delayed, the latter may not be. Using continuous thermodilution in a series of patients with angina and non-obstructive coronary arteries (ANOCA), including two habitual recreational drug users (cocaine and cannabis), CSFP is consistently associated with a vasoreactive epicardial coronary artery, i.e. endothelial dysfunction, but it bears neither noticeable relationship with epicardial artery calibre nor microvascular function. Hence, it is possible to conclude by process of elimination that unprovoked CSFP is purely a rheological manifestation, due to an interaction between the contrast media and a dysfunctional epicardial artery that has a “sticky” vessel wall, likely with glycocalyx degradation which is a hallmark feature of endothelial dysfunction. Hypothetically, when present, CSFP can be altered by changing contrast viscosities via dilutions.