Does a head injury reduce your tolerance to alcohol?

Increased risk. Alcohol can make it more risky for people with head injury since they may not be aware of body's warning signals.
It can. If the head injury was related to your drinking, and you are still drinking to excess, you need to strongly consider cutting down or stopping completely. Anything that damages the brain, including traumatic brain injury, aging and other brain diseases (like alzheimers) makes alcohol more toxic. If you are getting more drunk with less alcohol, your body is giving you a message. Listen to it!