Police in Tambon Nakhao in Chiang Khan district said Assada Ma-uan admitted killing Noon Ma-uan, 83, at their house in Moo 12 village.
Police quoted Assada as saying he’d taken three tablets of meth and became furious when his grandfather spurned his request for phone-credit money.
The younger man allegedly shot his elder in the chest with a homemade shotgun.