What happens if you swallow head lice ?

Probably nothing: Probably nothing, but I would avoid swallowing any bugs,

Wait what?: Hopefully you're referring to accidental, coincidental, or suspected ingestion, which would mean only a few of the tiny creatures made it in. While I'd advise against intentional ingestion, rest assured that we are designed to handle unwanted critters--the highly acidic stomach environment cannot be survived. Gross as it sounds, they'll be reduced to protein particles and eventually flushed out.

Head lice: It will most likely be killed in the digestive system.