A 51-year-old gentleman, who regularly smoked cannabis, presented with chest pain and diaphoresis. He was haemodynamically stable. ECG showed ST depression, inferiorly, and 1 mm ST elevation in lead aVR. Emergent coronary angiography showed thrombotic occlusion of the left main coronary artery (LMCA), the dominant RCA provided Rentrop grade II collaterals to the LAD. The LMCA was successfully reopened by deployment of a bare-metal stent. Animal heart models suggest that endogenous cannibinoids may cause ischaemic preconditioning. This case suggests that the severity of ischaemia, and hence ECG changes and haemodynamic consequences following an acute occlusion of the LMCA, can be ameliorated by coronary collateralisation and possibly by preconditioning of the myocardium.