Raspberry ketone: Where can you buy it, and are there any studies on it?

Wednesday's repeat episode of Dr. Oz has web surfers desperately hunting down information on the supplement raspberry ketone.

While some weight-loss supplements contain raspberry ketones, they also contain copious amounts of caffeine, which you don't want if you're already washing down your supplements with coffee or tea.

And when buying any sort of supplement, it's best not to buy in bulk, since what works for one person may be useless for another.

In the month since the Dr. Oz Show first aired, supplment manufacturers have ramped up production of raspberry ketones and they're much easier to find -- with or without other ingredients mixed in. Last month only a few places offered raspberry ketones and they were sold out in minutes. Some examples we've found today:

Vitacost.com has quite a few supplements offered, and like we said, some contain caffeine, which in small amounts can boost metabolism. But too much caffeine can leave you jittery and raise blood pressure -- never a good thing for people who already are overweight. Best deal: Vitacost Raspberry Ketones Featuring Raziberi-K, for $8.49. (There are no other ingredients in this product.)

Vitaminshoppe.com has some products that contain raspberry ketone but they also contain other ingredients. Best deal: Top Secret Nutrition L-Carnitine Plus Raspberry Ketone, for $23.99. L-carnitine is popular with bodybuilders, since the supplement claims to help turn fat into energy. But it can produce heartburn if you're sensitive.

GNC.com has some supplements that feature raspberry ketone, and many are bundled with other purported fat burners. Best deal: Hi-Tech Pharmeceuticals Raspberry Ketones, for $21.99, which is just raspberry ketone.

Hi-Health.com, which advertises toward the end of the Dr. Oz Show every day, seems to jump on the good doctors recommendations and features them on the front page of its website. The site features two products that contain it. Best deal: Ultra Plan Raspberry Ketone Plus Rhodiola, for $24.99. Rhodiola is a flowering plant that is said to improve mood and energy.

But should you even be taking raspberry ketone? The expert who was on his show, Lisa Lynn, is a fitness professional who also sells supplements on her LynFit website, and yes she also sells a weight-loss product that contains raspberry ketones, which she spells "keytones."

Have there been any credible, long-term studies on the effectiveness or safety of raspberry ketone? Well, if you're a mouse, you're in luck. In 2005, a Japanese study was performed on lab mice showing that raspberry ketone had similar properties to capsaicin and synephrine, two compounds that have been shown to boost metabolism and thus burn "white" fat, the type of body fat you want to get rid of. A similar South Korean study on lab mice in 2010 showed some fat-burning properties of raspberry ketones in the little critters.

But we're not mice and we haven't found any studies on humans for effectiveness or safety.

As for safety, raspberry ketone is used in the production of raspberry flavored foods and raspberry-scented products, and in 1965, the FDA labeled raspberry ketone "generally recognized as safe" or GRAS.