I used to take the pills even if it has just been expired.

Most likely it still works. If it doesn't work, just dump it. Also find out the chemistry information of the drug, whether it is an unstable molecule. But for the liquid, it might touch air which contain oxygen. Oxygen may accelerate the alteration of the medicine.

The company usually can not determine the precise timing when the drug could expire. They can only estimate. Whether the molecule of the drug has been changed or not, or even it become a toxic compound after the date of expiration, it also depends on the way how you store it, such as cap tight, separately from air, room temperature and in the refrigerator.