One, it may be dangerous and two, it is very very ineffective. Researchers found that the venom does not affect mucus membrane which is what the entire inside of your mouth is made of. The fault is, there may be cuts, scrapes, blisters, cavities, chapped lips, or many other imperfections in your mouth that will pass the venom into your blood stream. Also, your mouth can only produce a fraction of the amount of suction that a mechanical suction device can and some people have slowed down transportation by taking time to suction by mouth, which is a big mistake. Check with your local protocol about constrictor bands.
Most doctors agree that it is ineffective and dangerous.
Some people often try to include it in the treatment in a effort to do more. Definitely do NOT use the constrictor band if you don't know precisely what you are doing. Don't go slicing up the patient with the lance. It does not effectively increase the amount of venom that will be suctioned and usually only adds to the injury. Take a good look at the snake.