Aliens, who have invaded earth for a precious resource, are defeated by the power of specialised medicine from space.

Pre-emptive hint >! Definitely B grade pre-2010 !<

Edit - Going to sleep shortly. Will check in the morning. Here’s some more hints.

>! lead actor is rather Groovy !< >! the aliens love eating human head almost as much as wood !< >! the main character is a chiropractor !< >! It was released in 2005 !<