Merck & Co.'s Type 2 diabetes drug Januvia, approved by the FDA last Fall, has been a winner for the pharmaceutical company. It has "racked up surprisingly strong sales," Reuters reports. But it's also stirred a storm of controversy over side effects and even serious health risks associated with this once-daily tablet, sometimes referred to as "oral Byetta."

To explore the issue, let's start at the beginning. What exactly is Januvia and how does it work?

In addition to insulin, our bodies release certain "incretin hormones" called GLP-1 and GIP. In a healthy body, the ideal amount is released from your gut in response to the amount and type of food you ingest. The role of these hormones is to "increase insulin response to a meal" by suppressing glucagon release (which is good, since glucagon's job is to raise your blood glucose). In people with Type 2 diabetes, GLP-1 and GIP levels are below normal, and -- big AND -- they are intercepted by an enzyme called DPP-4. This is where Januvia (sitagliptin) comes in. It works to inhibit the DPP-4 that breaks down the GLP-1 and GIP.

So far, the only other drug on the market that aids (or rather mimics) the action of GLP-1 is Byetta, aka exenatide, an injectable drug which over half a million people are now taking, according to authorities. (There's also Januvia's up-and-coming rival from Novartis, called Galvus — still held up under FDA review).

These drugs are part of an emerging class of "incretin-based" therapies that block the DPP-4's destructive function, allowing the incretins to live a long and happy life — and hopefully fulfill their mission of recharging your beta cells. What this means for patients is better regulation of blood glucose levels, since the incretins trigger beta cell insulin secretion and impede glucose production by the liver.

The big selling point of Januvia was that it would trump all the other Type 2 diabetes drugs in terms of "tolerability," meaning you feel better on it, don't gain weight, don't tend to go low (hypoglycemia), and don't experience indigestion, heart problems, swelling, or other uncomfortable side effects. Or do you...?

As more and more people have now tried this new "wonder drug," they're beginning to share their experiences, good and bad. And the discussion is heating up. Some are delighted with the improved glucose control they can achieve — without injections — while others complain of headaches, nausea, and swollen feet and joints. Januvia has also been associated with a potentially fatal skin condition called Stevens-Johnson Syndrome, and serious allergic reactions. But these serious reactions are very rare, and the company says no "causal link" between the drug and the conditions has been established.

All I know is that the posts I have written on my web log about Januvia have become veritable boxing rings for patients duking it out over Januvia. A few examples:

Pro -

"I take Januvia and have for 6 months. I have no side effects. I have experienced a decrease in appetite. I have had no headaches or respiratory problems. My BG has gone down. It is a great drug." -- Mike

"Januvia has been a positive thing for me. My sugar levels dropped from 240+ to 110 +/- 10 after fasting. I have taken 100 mg once per day for a month... I am not as hungry as before usage." -- Jim K. L.

"I've been on Januvia for a little over three months and have lost over 15 pounds which I had put on with Actos. I've experienced no side effects, other than I am not hungry all of the time. For me it has been very effective..." -- Bill

"I am substantially less hungry then I have felt in years ... and my BG hovers around 100 - 120 between meals/fasting, and 120 - 160 for a few hours after a heavy carb meal. The usual BG spikes of 180 - 200+ are completely gone." -- Earl

Con -

"Prior to taking Januvia my BG was in the 160 to 180 range. Now it has gone up into the 260 to 300 range. I feel OK but this can't be right." -- Tom Peck

"Januvia is no picnic -- from the first time I took it felt as if I had a fever would shiver as if I was freezing after a week added getting dizzy to the mix spent the day in the hospital my enzymes were all messed up but got better as time went on within 2 days of stopping Januvia all symptoms went away." -- Marty

"Januvia seems to have raised my blood sugar to unacceptable levels. Although I was put on it because my numbers were too high, the Januvia seemed to raise them even higher, into the 300's. Not good! Dr. told me to stop it for a few days and try it again. I did. Numbers dropped way down to 80 to 110, although I was still having some high numbers in the 200's, particularly in the morning..." -- Julie

"I used Januvia for a month and a half. It worked well at first. Then my right knee swelled, my left foot I could not walk on. When I stopped taking Januvia swelling went away. It worked well at controlling my blood sugar levels, but too much pain. Oh... I never injured my knee or foot." -- Kathleen S.

Apparently some medical industry insiders have been overheard calling Januvia "the new oral Byetta" — offering the same glucose control effects, but easier for doctors to prescribe and teach (no injections necessary), and therefore easier for patients to "remain compliant."

"Calling it 'oral Byetta' is a vast overstatement," says analyst David Kliff of Diabetic Investor. "Byetta blows it away for effectiveness ... and if you add in the weight loss effects and possible slowing of beta cell destruction, Byetta's a much better drug. And the fact that it's injectable? I don't view as that as a negative."

Maybe not, but many people surely do. Still, the bigger issue remains how patients are reacting physically to Januvia — whether or not they experience intolerable side effects. And that, clearly, is quite a mixed bag right now.

Want to know more about what patients think of Januvia? Check out the discussion forums at my site, DiabetesMine.com, or over at Diabetes.Blog.com.

Amy Tenderich hosts the popular web log www.diabetesmine.com and is co-author of the new book, "Know Your Numbers, Outlive Your Diabetes," the first-ever hands-on guide to achieving a long and healthy life with diabetes. Find out how to get your free copy today.