12.5mg is a very, very low dose. By comparison, I'm on 75mg and have been as high as 200mg. If you look at the plasma half life of Metoprolol, at the end of the day, there is very little remaining in you blood. Are you taking it more than once a day? With Metoprolol Tartrate (the standard release kind) taking it only once a day creates these peaks and valleys as opposed to a more level dose.
I have to agree with tom_h. I have taken Metoprolol Succinate (Toprol XL) for many years. I went as high as 150 mg, but it never had really any effect on my BP - that is why it was originally prescribed for me and I dropped it because it was useless for that purpose. Now I am taken it again (50 mg) for Pac's/Pvc's and it reduces the number I have each day.