While my daughter was on Pentasa. and then Asacol, and then Pentasa again, her levels of vitamin D, iron, and B12 all took a nose dive in just weeks.
I insisted on blood test before starting mesalamine, and then we checked weekly progress. Quite frankly, I was shocked to see how fast her hemoglobin (due to plummeting iron stores) could drop while on mesalamine.
She also developed an abnormally low white blood cell count, otherwise known as leukopenia, while on mesalamine. She came down with mono while this happened, so we haven't ruled out mono as the cause of leukopenia. All I can say is that the leukopenia vanished within two days of stopping Pentasa, but of course she still had the mono. Also, if you check medical literature, you will see multiple warnings saying that leukopenia is not typical of mono, and that mono tends to cause higher than normal, not lower than normal, white blood cell count.
If I hadn't insisted on weekly blood tests while she trialed mesalamine, we wouldn't have even known about the leukopenia, much less the drops in D, B12, iron...as an example, within six weeks of starting mesalamine, her hemoglobin went from a healthy 14.8 to an anemic 11.9.
I'm not saying you shouldn't give the med a try...I'm just saying that blood levels need to be checked weekly in the first few weeks you start it, because not all reactions are obvious, right away.