Punnami Naagu (1980)

Naagulu (Chiranjeevi), a snake charmer, falls in love with Menaka, whose adopted brother Raju (Narasimha Raju) is in love with Naagulu's cousin Laxmi (Rati Agnihotri). Naagulu's father has been feeding mixing snake poison in his food since childhood, making Naagulu immune to any snake bite. Every full moon, Naagulu behaves like a cobra and searches for a woman and every woman he meets dies from his poison (Wikipedia)