The fifth precept is a bit different from the other four. It is not a gross demeritorious deed in itself, for consumption of intoxicants is not directly harmful to others. However it does provide the potential for heedlessness, negligence, and the potential to commit any of the ten demeritorious deeds, which is why it is a precept. Intoxication disrupts mindfulness, stillness, and hinders one's dhamma practice, despite many deluded claims to the contrary.
Note that according to the Vinaya, the fifth precept was the last of the five precepts to come into effect, after a monk received alcohol as alms, consumed it and consequently collapsed on the ground. He had to be carried back to his monastery and missed a dhamma talk by the Buddha. From that point on, the Buddha forbade his monks to consume or knowingly accept alcohol.
Ajahn Brahm has said that if one finds it hard to keep five precepts, keep only two: to not harm others and to not harm oneself. The first four precepts are about not harming others and the fifth one is to not harm yourself.
quidquid Latine dictum sit altum videtur
(Anything in Latin sounds profound.)