"... Toprol-XL produced significantly higher total beta-1
blockade over 24 hours than immediate release metoprolol
(Lopressor) ... In comparison to regular metoprolol, blood
levels of Toprol-XL show lower peaks, longer time to peak
level, and significantly lower peak to trough variation.
Peak blood levels after once-daily Toprol-XL are only 1/4
to 1/2 as high as peak blood levels after an equal dose of
metoprolol ..."