There could be a couple of things causing this to happen. You mention that it uses allot more gas so I am leaning toward a leaking injector. But It could also be compression loss from a bad head gasket or worn piston rings. First I would do a compression test and cyl leakage test on the motor to test the compression. Then pull the injector to check the condition of it. If you keep driving the vehicle you will damage the catalytic converter due to excess hydrocarbons being introduced.
Check the plug and compare it to the other plugs. The plug will be darker then the others from fuel foul. Next check it electrically with a noid light. If the pulse is good then you will need to remove it. Swap it to a different cyl, See if the problem goes to the other cyl. If so, replace the injector.