Stimulants in therapeutic doses, such as those given to patients with ADHD, increases ability to focus, vigor, sociability, libido and may elevate mood. However, in higher doses stimulants may actually decrease the ability to focus, a principle of the Yerkes-Dodson Law. In higher doses stimulants may also produce euphoria, vigor, and decrease need for sleep. Many, but not all, stimulants have ergogenic effects. Drugs such as ephedrine, pseudoephedrine, amphetamine and methylphenidate have well documented ergogenic effects, while cocaine has the opposite effect. Neurocognitive enhancing effects of stimulants, specifically modafinil, amphetamine and methylphenidate have been documented in healthy adolescents, and is a commonly cited reason among illicit drug users for use, particularly among college students in the context of studying.
In some cases psychiatric phenomenon may emerge such as stimulant psychosis, paranoia, and suicidal ideation. Acute toxicity has been reportedly associated with a homicide, paranoia, aggressive behavior, motor dysfunction, and punding. The violent and aggressive behavior associated with acute stimulant toxicity may partially be driven by paranoia. Most drugs classified as stimulants are sympathomimetics, that is they stimulate the sympathetic branch of the autonomic nervous system. This leads to effects such as mydriasis, increased heart rate, blood pressure, respiratory rate and body temperature. When these changes become pathological, they are called arrhythmia, hypertension, and hyperthermia, and may lead to rhabdomyolysis, stroke, cardiac arrest, or seizures. However, given the complexity of the mechanisms that underlie these potentially fatal outcomes of acute stimulant toxicity, it is impossible to determine what dose may be lethal.
Assessment of the effects of stimulants is relevant given the large population currently taking stimulants. A systematic review of cardiovascular effects of prescription stimulants found no association in children, but found a correlation between prescription stimulant use and ischemic heart attacks. A review over a four-year period found that there were few negative effects of stimulant treatment, but stressed the need for longer term studies. A review of a year long period of prescription stimulant use in those with ADHD found that cardiovascular side effects were limited to transient increases in blood pressure only. Initiation of stimulant treatment in those with ADHD in early childhood appears to carry benefits into adulthood with regard to social and cognitive functioning, and appears to be relatively safe.
Abuse of prescription stimulants (not following physician instruction) or of illicit stimulants carries many negative health risks. Abuse of cocaine, depending upon route of administration, increases risk of cardiorespiratory disease, stroke, and sepsis. Some effects are dependent upon the route of administration, with intravenous use associated with the transmission of many disease such as Hepatitis C, HIV/AIDS and potential medical emergencies such as infection, thrombosis or pseudoaneurysm, while inhalation may be associated with increased lower respiratory tract infection, lung cancer, and pathological restricting of lung tissue. Cocaine may also increase risk for autoimmune disease and damage nasal cartilage. Abuse of methamphetamine produces similar effects as well as marked degeneration of dopaminergic neurons, resulting in an increased risk for Parkinson's Disease.