Is the prohibition against eating onions and garlic when going to the mosque a prohibition of unlawfulness (tahreem)? And is one who has eaten them prevented from entering the mosque? And does cleaning the mouth with a brush and toothpaste after eating them negate this prohibition?

The prohibition of entering the mosque for one who has eaten garlic and the like is a matter of scholarly disagreement. The preponderant view is that it is a prohibition (tahrim), not merely a dislike (karahah). This is supported by the action of the Prophet, peace and blessings be upon him, who commanded the expulsion from the mosque of anyone from whom the smell of garlic was detected. This is also the view of some contemporary scholars, such as Ibn Baz. This prohibition ceases when the offensive odor completely disappears by any means, but it is common for the smell of garlic not to vanish merely by using toothpaste and a brush, because its source is from within the body.