A March report from pharmacy benefit manager Express Scripts revealed that U.S. prescriptions for adult attention deficit hyperactive disorder drugs rose from 1.7 million in 2008 to 2.6 million in 2012 -- a 53% increase. That study, which analyzed 15 million privately insured people under the age of 65, also uncovered an 85% increase in use of ADHD drugs among women between the ages of 26 and 34.

Some medical experts believe those figures indicate that ADHD drugs are being overprescribed, while others argue that prescriptions have risen due to an increased awareness of the disorder. Despite the controversy that surrounds these medications, health-care investors should be aware of the key differences between ADHD drugs, and three companies in the field that are providing treatments: Shire (NASDAQ:SHPG), Johnson & Johnson (NYSE:JNJ), and Alcobra (NASDAQ:ADHD).

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Shire: Adderall, Vyvanse, and Intuniv
Shire's three main ADHD drugs -- Adderall XR, Vyvanse, and Intuniv -- generated $519 million in combined sales in the last quarter, accounting for 40% of the company's top line.

Sales of Adderall XR fell 15% year over year to $85.1 million due in part to generic competition, but sales of Vyvanse, which is approved for both children and adults, rose 18% to $351.2 million. Sales of Intuniv climbed 6% to $82.3 million.

Adderall and Vyvanse are both classified as stimulants, due to containing amphetamines, and are classified as controlled substances by the Drug Enforcement Administration. Stimulants may cause heart problems in patients with pre-existing cardiovascular conditions.

Intuniv was Shire's answer to concerns regarding stimulant ADHD drugs. The product is not a stimulant and mainly competes against Eli Lilly's Strattera, another nonstimulant ADHD drug.

Johnson & Johnson: Concerta and Risperdal
Johnson & Johnson's Concerta is also a stimulant. Concerta's main patents won't expire until 2017 and 2018, but generic versions have nonetheless been approved. After a court ruled that one of Concerta's patents was invalid in 2009, J&J reached agreements with Watson (now known as Actavis), Impax Laboratories, and other drugmakers to manufacture generic versions of the drug. Those generic versions have taken a bite out of J&J's pharmaceutical revenues.

Last quarter, Concerta sales fell 41% year over year to $150 million, due to competition from generic drugs. In 2013, sales fell 27% to $782 million -- a steep drop from the $1.3 billion in sales that it generated back in 2010.

That decline reduced Concerta from 5.9% of J&J's pharmaceutical revenue in 2010 to just 2.8% in 2013. However, J&J hasn't given up on protecting its remaining Concerta sales with more legal action. In 2013, it sued Osmotica Pharmaceutical's Hungarian Unit over generic Concerta, but later dropped the case. Earlier this month, it sued Mylan for the same reason.

Johnson & Johnson's involvement in the ADHD market doesn't end with Concerta, however. Its antipsychotic drug Risperdal has also been prescribed to children with ADHD in the past. That practice, along with several other controversial uses, led to a $2.2 billion fine for J&J last November for the improper off-label promotion of the drug to children and adults.

Alcobra: A newcomer in nonstimulant ADHD drugs
Last but not least, biotech Alcobra also works in this space. Alcobra's experimental drug Metadoxine Extended Release, or MDX, is a nonstimulant treatment being tested for predominantly inattentive ADHD, which accounts for 40% of all ADHD cases, and Fragile X syndrome, a genetic syndrome that causes autism in boys. MDX was shown to be effective and well tolerated in two phase 2 studies of adults with ADHD. It has begun enrolling patients in a phase 3 trial, with completion expected in the second half of this year.

Certainly, Alcobra is an interesting stock to watch. Nonetheless, this is a small, clinical-stage biotech, so an investment here is comparatively risky.

Foolish final thoughts
The market for ADHD treatments might initially appear saturated with generic drugs, such as cheaper versions of Shire's Adderall and J&J's Concerta.

However, Shire and Alcobra show us that there's still room for improvement in the market. Shire's Vyvanse shows that new formulations of stimulants can be less habit-forming than older ones, while demand for non-stimulants like Intuniv and Strattera remain high. That means there's still room for newcomers like Alcobra to find space in this market with newer nonstimulant treatments.

In conclusion, investors should keep a close eye on these companies and see which of their drugs benefit the most from the sharp rise in ADHD prescriptions over the past few years, especially if that growth continues.