[3] The frontal, supraoculars, and parietal shields are large, but those on the snout are small and irregular.

[2] The color pattern is grayish with heavy brown mottling, overlaid with a double row of large dark spots.

It has an irritable disposition and will vibrate its tail when annoyed,[2][8] a behavior it has in common with other pit vipers, especially rattlesnakes of the genera Crotalus and Sistrurus.

[9] Bites from H. hypnale, although previously thought to be innocuous, are now known to cause serious complications such as coagulopathy and acute renal failure (ARF).

[9] As of November 2016, an antivenom is currently being developed by the Costa Rican Clodomiro Picado Institute, and clinical trial phase in Sri Lanka.