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A small study done by Dr. Tillmann Kruger of Germany demonstrated that Botox injections into the vertical frown lines of the forehead, also known as glabellar lines, may relieve depression.
The theory behind the improvement in depression with Botox treatment is that the negative facial expression that is conveyed by the vertical lines caused by frowning, reinforces negative inner emotions. By taking away the ability to make the frown lines, or by decreasing the appearance of the glabellar frown line, the feedback loop is disrupted.
The patients were aged 25 to 65 and were on stable antidepressant treatment. They also had a moderate to severe glabella line. The end point was a score the Hamilton Depression Rating Scale (HAM-D17, an expert-rated instrument) six weeks after treatment.
The treatment worked in both men and women, with higher doses needed in men due to stronger male frowning muscles.
Limitations of the study are that the sample size (number of people in the study) was small. Additionally, it's difficult to have a legitimate placebo group. A person knows if they do or do not have the ability to frown.