Bathrooms are for relieving oneself, not for eating and drinking, so it is not appropriate for the Muslim to enter them except for the purpose of relieving himself, and when he enters the bathroom he should refrain from eating and drinking until he comes out.
If a person eats and drinks inside the bathroom when there is no need to do so, then he has done something which is contrary to sound human nature (fitrah).
Shaykh Ibn Uthaymeen (may Allah have mercy on him) was asked about the ruling on eating and drinking in the bathroom.
The bathroom is a place for relieving oneself only and one should not stay there longer than is necessary. Eating or doing other things there requires staying there for a length of time that is not appropriate.
End quote from Majmoo al-Fataawa, 11/110
And Allah knows best.
Reproduced from Islam QA