Supplements are often taken for anxiety, depression, insomnia, cognitive decline, and dementia. In the subsections potential indications are considered along with various ratings of scientific evidence. Keep in mind that prescription medications by comparison would generally get "A ratings" with Excellent Scientific Evidence for efficacy.
I recommend that supplements be taken with caution because they are not tested for safety or regulated by the FDA like prescription medications, yet they can also have serious side effects, cause birth defects, get into breast milk, and interact with other medications and supplements along with other potential complications. In addition if a supplement is self-prescribed, you may have misdiagnosed your condition and might be putting yourself in jeopardy of worsening symptoms even if the given product might normally help for a given condition. Thus, I believe supplements are best utilized when recommended by a doctor who is familiar with their use and who has diagnosed the condition for which they are intended, and can monitor for safety and progress.