Switched from generic Toprol XL to generic Lopressor

Because of the recent shortage of metoprolol succinate, my general practitioner refilled my prescription as metoprolol tartrate, but without informing me of the change. I was previously taking 25 mg in the am and 12.5 mg in the evening of the generic Toprol XL. Now I am taking 25 mg in the morning of generic Lopressor, and another 25 in the evening, but less than a full 12 hours apart. For example I will take the 25mg at 7 am, another 12.5 at 2pm and another 12.5 at around 6 or so.I have had breakthrough tachycardia, mostly when I wake up in the morning or a few hours after. Could this be caused by the change in medication? The pharmacist at Walgreens told me that the generic Lopressor only lasts 10-12 hours, as opposed to 24 for the XL. My question is, if I was taking 25mg in the am and 12.5 in the evening of Toprol XL, and now I am taking 25 of Lopressor in the am and pm, is this equivalent, or will my body have to adjust to a change in dosage, and might I experience side effects? Thank you.

[Sandra Y, RPh]

The mg dose is the same, so you should transition without needing adjustment. The difference between Lopressor and Toprol XL is the release time (12 hrs vs 24 hrs). Since you're still taking your doses twice daily, you should experience the same results. If not, the generic Toprol XL 25 mg is available again.