Discussions about antidepressants and weight gain are all over the Internet, from scholarly articles and studies to blogs by those who are experiencing obesity from taking them.

“In one single year on Zoloft my weight doubled from about 100 lbs to about 200 lbs,” writes one blogger. “No doctor at the time would believe me that it was the drugs and not something I was doing. I was doing what I always had done. I had always been very athletic and had always eaten a super clean diet. I was pushing 40 at the time and still fit comfortably in my clothes from high school pre-SSRI.”

Another blogger said she gained 100 pounds after taking an antidepressant. During that time she hired a personal trainer and a dietician. Nothing, she said, stopped the weight from piling on.

“The weight started to pack on in the first year on antidepressants. Five years later my thyroid stopped working and over a period of seven years I gained 100 lbs,” she wrote. “I’ve never been able to lose a single pound while on antidepressants. Not even when I went down to 1,200 calories a day and put personal training on my credit card when I was on awful Paxil.”

The U.S. Centers for Disease Control reports approximately one in 10 Americans takes antidepressants. The drugs are prescribed for various reasons, including anxiety, depression, obsessive compulsive disorder, panic disorder, hot flashes and pain. Over the past two decades, use of these drugs has risen 400 percent, making them the most frequent prescription for Americans ages 18 to 44.

At the same time, the CDC reports that obesity rates have also gone up. More than one-third of U.S. adults is considered obese.

So what causes people to gain weight while taking these drugs?

No one really understands. In a guest commentary on CNN.com, Emory University Medical School psychiatrist Dr. Charles Raison said antidepressants work by interacting with various chemical receptors in the brain.

“Two of these receptors are especially associated with a proclivity toward weight gain when blocked. These are the muscarinic acetylcholine receptor and the histamine receptors,” he said. “Antidepressants that block these receptors have more weight gain associated with them than do antidepressants that avoid them.”

Raison said two antidepressants responsible for quite a bit of weight gain include the selective serotonin reuptake inhibitor Paxil and the tricyclic antidepressant Elavil.

Another relatively new antidepressant, Remeron, is also associated with significant weight gain.

Dr. Roy Perlis, senior author of a recent study published in the Journal of American Medical Association’s Psychiatry, used health records from 22,610 adults treated with antidepressants, recording their weights over the course of a year. Researchers found on average, people taking antidepressants will gain a very modest amount of weight — between half a pound and perhaps two pounds if they stayed on the medicine for about a year. But some people will gain much more.

Perlis, a psychiatrist at Harvard Medical School and director of the Center for Experimental Drugs and Diagnostics at Massachusetts General Hospital in Boston, said while the study can’t absolutely prove antidepressants were responsible for weight gain, newer medications with fewer side effects are needed.

Another study published in the Journal Public Library Of Science found a strong association of obesity with antidepressant use. The study found women taking antidepressants were 14 percent more likely to be overweight and 71 percent more likely to be obese than women not taking the medications.

An article on WebMD states that up to 25 percent of people taking antidepressant medications tend to gain weight.

“This is a phenomenon that I first noticed years ago when Prozac first came on the market. It didn’t initially show up in the clinical trials because most of them were eight to 12 weeks in length, and the weight gain generally occurs with longer use. But it’s definitely one of the side effects of this and other antidepressant medications,” said Dr. Norman Sussman, a psychiatrist and associate dean for postgraduate medical programs at the NYU School of Medicine.

And, in the August edition of the Medical Chronicle, researchers found some of the medications can stimulate appetite, increasing cravings for certain foods. However, the study also found some people gained weight even though their eating habits did not change, which might mean it’s possible these medications can alter your basic metabolism and interfere with central nervous system functions.

SSRIs aren’t the only medications that can have weight gain side effects. Tricylic antidepressants, MAO inhibitors, drugs that treat cholesterol, diabetes, cancer and even acid reflux have weight gain listed as a side effect.

Experts say no one should ever stop taking their medication without first talking with their doctors. Stopping suddenly can be very dangerous and cause horrible withdrawal symptoms. In addition, don’t mix weight loss pills with antidepressants and do not go on starvation diets. Instead, keep in close contact with your physician and find a solution that works best for you.