Praise be to Allah.
It was proven that the Prophet (blessings and peace of Allaah be upon him) forbade drinking from the mouth of the waterskin. Narrated by al-Bukhaari (5628) and Muslim (5629) from the hadeeth of Abu Hurayrah and Ibn ‘Abbaas (may Allaah be pleased with them).
The siqa’ (translated here as waterskin) is the vessel in which water is put and it has a mouth from which it is drunk.
The scholars (may Allaah have mercy on them) mentioned a number of reasons for this prohibition:
That what is inside the waterskin cannot be seen, and there may be an insect or snake inside it which could cause harm or annoyance, as it was narrated that a man drank from the mouth of the waterskin and a snake came out of it.
This reason does not apply with regard to drinking from bottles nowadays, because what is inside them can usually be seen.
That the person who drinks from the mouth of the waterskin may not be able to control the flow of water, so he will get more than he needs and it will choke him or make his clothes wet.
This reason does apply with regard to those who drink from bottles, as we see happen with a lot of people.
The prohibition is there so that his saliva will not get on the mouth of the bottle or be mixed with the water inside it, or so that his breath will not get on the mouth of the bottle and thus put others off. This may be a cause of the spread of disease.
This reason is also applicable to those who drink from bottles, but it applies to the one who touches the bottle with his mouth. But if he pours from it and does not touch it with his mouth, there ts nothing wrong with it.
Similarly, it applies in cases where someone else will drink from the same bottle after him. But if the bottle is just for him, there is nothing wrong with drinking from its mouth in that case.
It is not unlikely that the prohibition on drinking from the mouth of the waterskin was for all of these reasons, as was stated by Ibn al-‘Arabi, Ibn Abi Hamzah and others (may Allaah have mercy on them).
See Fath al-Baari, commentary on hadeeth number 5628.
Some of these reasons, as stated above, apply to the one who drinks from a bottle, hence he should not drink from its mouth, especially if someone else is going to drink from the bottle too.
And Allaah knows best.