Thunderclap headache (TCH) can have several causes of which subarachnoid hemorrhage (SAH) is most common and well known. A rare cause of TCH is the reversible cerebral vasoconstriction syndrome (RCVS) which is characterized by a reversible segmental vasoconstriction of the intracranial vessels. We describe two patients with TCH due to RCVS and the probable precipitating factor, namely, cannabis and an anti-migraine drug. In RCVS, cerebrospinal fluid examination is (near) normal, in contrast to SAH and (primary) cerebral vasculitis. Brain MRI may be normal or shows infarction. MRA can demonstrate vasoconstriction of the great arteries, but a normal MRA does not rule out the diagnosis. Caliber changes on cerebral angiography cannot adequately differentiate between RCVS and vasculitis. Calcium-channel antagonists may be a good therapy and repeated transcranial Doppler ultrasonography can be a reliable non-invasive investigation to monitor the effect of treatment and demonstrate reversibility of the vasoconstriction.