Are the directions on the bottles of medications a hard rule or is dosage more flexible?

I am not asking for medical advice.

All OTC medicines have directions that say something along the lines of: “take once every couple hours” but I was wondering what would happen/how bad it would be if I did not follow those directions?

For example, I was previously told to take 1 Zyrtec every 12 hours by a doctor for my mosquito bite allergy, but the bottle’s directions say not to take more than 1 tablet in 24 hours. Why did the doctor’s advice go against what the medication says? Does this mean the medication isn’t as effective when taken as directed by the directions on the bottle?

Thanks!