Real som tam will destroy most tourists but it's worth building up tolerance. Start with 'pet nit noi' or 'mai pet' to see how they make it. The vendors use bird's eye chilies and they don't mess around

Once you can handle that, try 'pet', then work up to 'pet maak'. I've seen grown adults crying after ordering pet maak thinking they could handle it. The version at Thewet Market will melt your face off but it's incredible if you can take the heat