I think this is something that everyone needs to know, yet is hardly discussed: Antibiotics can cause depression.

I became aware of this last January, when I took Flagyl (metronidazole) and started feeling probably the worst I have felt in my life. When nothing seemed to explain my change in mood, I realized that my antibiotic might be to blame, and sure enough, an Internet search revealed that other people have become extremely depressed--even suicidal--on metronidazole and many other antibiotics. With that knowledge, I asked my doctor (who had not heard of this side effect) if I could stop the medication early, and I soon felt better.

But what if I hadn't realized that antibiotics can cause depression? I might have made poor choices, thinking something else was causing my depression. Sometimes people commit suicide on antibiotics. For example, there have been so many cases of suicide on the acne medication Accutane that there was a Congressional investigation into the matter.

Still, however, the connection between antibiotics and depression is rarely mentioned in mainstream sources. When you search for this topic online, you'll find plenty of forum posts and mentions in long lists of antibiotic side effects, but you'll see hardly any mention in health writing or patient education. People know that antibiotics can cause digestive symptoms and yeast infections, but it's not widely known that they can cause depression. For example, when I told my pharmacist I was feeling depressed on Flagyl, she said "But it's an antibiotic--it can't be making you depressed."

I think it's essential for anyone who might take, prescribe, or dispsense an antibiotic (which means most of us) to be aware that antibiotics can cause depression and other psychiatric side effects. It's much easier to cope with antibiotic-induced mood changes when one knows that they are, in fact, antibiotic-induced.

Several factors might contribute to depression on antibiotics. First, there can be side effects from the medication itself. Additionally, there might be depression from the change in gut flora or from the loss of healthy bacteria (which serve functions such as producing and helping with the absorption of certain vitamins). It's been noted that gut bacteria can affect mood. A report by Lowry et al. in Neuroscience (2007;146(2):756-772) found that the soil microbe Mycobacterium vaccae essentially improved the mood of mice (as it had been found to do in humans when it was tested as a potential cancer treatment). In contrast, Gaykema et al. found that an intestinal Campylobacter jejuni infection made mice more anxious (Brain, Behavior, and Immunity 2004;18(3):238-245).

Just as we have come to appreciate the mind-body connection more in recent years, we may start to recognize the bacteria-mind connection. But we don't need to wait until that happens to acknowledge and make people aware of the antibiotic-depression connection.

Andrea Runyan can be reached at monandreamichelle "at" gmail.com or http://freeideasblog.blogspot.com/.