The medical community certainly spends a lot of time and money researching treatments for cardiovascular disease and cancer. It's not hard to understand why, as these two diseases were the two leading causes of death in the U.S. in 2010. However, researchers and science advocates are cutting themselves short if they don't realize another serious threat exists in the U.S.: mental disorders.

According to the National Institute of Mental Health, or NIMH, an estimated 26.2% of adults aged 18 and older, nearly 63 million people, will suffer from a diagnosable mental disorder in any given year. These diseases range from more moderate mental illnesses -- those that lifestyle changes and medicine can help relieve -- to the more severe mental illnesses that affect 6% of the population.

Today, I propose to look at the three most commonly diagnosed mental disorders courtesy of Discovery and see what, if any, medications are being used to counteract or lessen the effects of these disorders. Ultimately, 63 million people represents a huge patient subset, leaving plenty of opportunity for big pharmaceutical and biotech companies to make a meaningful quality-of-life impact.

1. Mood disorders

The most commonly diagnosed of all mental disorders is mood disorder, which affects about 21 million adults in the United States. Included in mood disorders is major depressive disorder, dysthymic disorder, and bipolar disorder.

Major depressive disorder is by far the most common, affecting nearly 15 million people, and is the leading causing of disability for people aged 15-44 according, to the NIMH. It's also more prevalent in women than in men. You will find nothing short of a couple of dozen possible treatments that can be used to help fight clinical depression, with none standing out more than Eli Lilly's (NYSE:LLY) Cymbalta, which was a $5 billion drug for Eli Lilly in 2012 and accounted for 22% of its total product sales. However, its patent on the drug is set to run out in the fourth quarter of this year and will almost certainly open up the door for a slew of branded and generic competition.

Also known as dysthymia, dysthymic disorder is a state of prolonged chronic depression that often has symptoms that are less severe than major depressive disorder. It affects a tad more than 3 million people in the United States. The most common treatment for dysthymia is a combination of antidepressants -- usually a selective serotonin reuptake inhibitor such as the now generic Lexapro, developed by Forest Laboratories -- and psychotherapy. This is actually great news for patients, because nearly all SSRIs are now off patent (Lexapro lost its exclusivity last year), making them incredibly cost effective.

Bipolar disorder is the final mood disorder characterized by major shifts in mood, energy, and ability to function, and it affects roughly 6 million people in the United States. Similar to dysthymia, medication and counseling are the two most common therapies used to keep this long-term disease in check. Perhaps one of the best-known treatments is Johnson & Johnson's Risperdal, which has long since gone to a generic form and is now manufactured by Teva Pharmaceutical (NYSE:TEVA).

If you take away anything from mood disorders, it should be that generic drugs rule the roost. Mood disorders are in desperate need of newer treatment options, but most big pharmaceutical companies lose any incentive to enter the fray when large generic competitors such as Teva offer treatments for a fraction of the branded drugs' cost. Vanda Pharmaceuticals was the latest to try its luck with major depressive disorder but failed in phase 2b/3 trials, when tasimelteon didn't hit its primary endpoint. Until further notice, generic-drug makers such as Teva are your best investment bet when it comes to mood disorders.

2. Personality disorders

Approximately 9% of the population exhibits some form of personality disorder, which, like mood disorders, comes in three common diagnoses: antisocial personality disorder, avoidant personality disorder, and borderline personality disorder.

Antisocial personality disorder is diagnosed in about 1% of the population and is characterized by a person's unwillingness to follow social rules and cultural norms. Most people with this disorder are unable to differentiate between right and wrong and show little remorse for their actions. Antisocial personality disorder is incredibly difficult to treat, because there isn't a drug specifically designed to deal with it, nor do all people who have it recognize they have a problem that needs treating. Commonly, a combination of counseling and medication, which can range from antipsychotics to antidepressants, is used.

Avoidant personality disorder is a lifelong health condition that affects more than 5% of the population and is characterized by a person's feeling of being shy, inadequate, and sensitive to what others have to say about them. The most commonly prescribed medication here would be generic forms of Eli Lilly's Prozac. Like many mood disorders, antidepressants and anti-anxiety medications work well to counter the social phobias often exhibited by people with this disorder, but few new medications have been developed. Again, that's a big win for generic-drug makers and the consumers, but also a disappointment in that no new meds are hitting the market.

The third common personality disorder is borderline personality disorder, or BPD, which affects about 1.6% of the U.S. population. People with this disease are often emotionally unstable and impulsive in nature. They also tend to have trouble carrying on interpersonal relationships with others. While BPD is often inherent at birth, patients can undergo a mixture of talk therapy and drugs to help alleviate symptoms. In the drug department, this could include any combination of anti-psychotics, antidepressants, anti-anxiety drugs, and mood stabilizers. One of the most effective anti-psychotics in this space has been Bristol-Myers Squibb (NYSE:BMY) and Otsuka Holdings' Abilify, which is used to treat bipolar disorder and schizophrenia. U.S. net sales of the drug topped $4.2 billion last year, and net sales in the first quarter added 10% compared with the year-ago period.

3. Eating disorders

The third most-commonly diagnosed mental disorder is eating disorders, which (not to sound like a broken record) fall into three primary categories: anorexia nervosa, bulimia nervosa, and binge-eating disorder. Women are three times as likely as men to develop anorexia nervosa or bulimia nervosa in their lifetime and 75% more likely to be diagnosed with binge-eating disorder than men, according to the NIMH.

Of the mental disorders we've discussed here, perhaps none benefits the least from a medication perspective than anorexia nervosa, a condition characterized by being chronically underweight and malnourished. Because of the serious effects being malnourished can have on the body, hospitalization may be required to stabilize normal body functions, but long-term treatment involving individual, group, and family therapy is often the remedy. If any drugs do tend to be prescribed, it tends to be an antidepressant such as Prozac.

In bulimia nervosa, better known simply as bulimia, patients tend to eat, or in some cases binge-eat, and then purge the food from their bodies before they've had a chance to absorb the proper amount of nutrients. Others may choose a non-purging method such as fasting to ensure no weight gain. Either method tends to leave bulimic patients as anorexic and malnourished, which can lead to life-threatening health issues. Here again, medication is rarely as helpful as talk therapy in treating the disease. The only FDA-approved therapy to treat bulimia nervosa is Eli Lilly's antidepressant Prozac, but even that won't have much effect without a heavy dose of individual, group, and family support.

The third eating disorder is binge-eating disorder, which is characterized by regularly consuming large quantities of food, which often results in excessive weight gain. Even the people who recognize these urges are wrong can't stop themselves from overeating. As with the other eating disorders, talk therapy is an important aspect of treatment, but medication can also play a crucial role. Antidepressants are a commonly prescribed tool used to improve a patient's mind-set, but I could see chronic weight-management drugs such as Arena Pharmaceuticals' (NASDAQ:ARNA) Belviq and VIVUS' (NASDAQ:VVUS) Qsymia potentially playing a role in treating binge-eating disorders in the future.

With binge-eating patients often not in the best mental state, Belviq and Qsymia in combination with an antidepressant could help shed some initial weight for patients who are clinically considered obese. Simply losing a few percentage points of body weight could be enough to completely change that person's state of mind and turn his or her life around. With Belviq presenting the better safety profile in trials and Qsymia boasting the more sizable weight loss, both drugs could have an honest shot at being weight-control blockbusters.

Foolish roundup

Although they often fly under the radar, mental disorders affect a big enough subset of our population that researchers should be spending more time coming up with treatments to treat these diseases. Until such time as we see research in this area spike, the aforementioned companies have relatively unopposed revenue streams and, as such, could make for an intriguing investment opportunity.