Use 'have a screw loose' in casual conversations, but not in formal or serious situations. My neighbor talks to plants; I think he may have a screw loose.
Pro Tip 2/3
Not Offensive
It is a humorous phrase and not usually meant to offend, but be careful as some may find it insensitive. She believes in aliens and ghosts; some say she has a screw loose.
Pro Tip 3/3
Not Literal
The phrase is a metaphor and does not refer to actual screws or repair work. He juggles fruit at the bus stop like he's got a screw loose.