<P> Methylphenidate is a stimulant drug that is often used in the treatment of ADHD and narcolepsy and occasionally to treat obesity in combination with diet restraints and exercise . Its effects at therapeutic doses include increased focus, increased alertness, decreased appetite, decreased need for sleep and decreased impulsivity . Methylphenidate is not usually used recreationally, but when it is used, its effects are very similar to those of amphetamines . </P> <P> Methylphenidate acts a norepinephrine - dopamine reuptake inhibitor, by blocking the norepinephrine transporter (NET) and the dopamine transporter (DAT). Methylphenidate has a higher affinity for the dopamine transporter than for the norepinephrine transporter, and so its effects are mainly due to elevated dopamine levels caused by the inhibited reuptake of dopamine, however increased norepinephrine levels also contribute to various of the effects caused by the drug . </P> <P> Methylphenidate is sold in many forms, its most common trade name is the oral tablet Ritalin . Other versions include the long lasting tablet Concerta and the long lasting transdermal patch Daytrana . </P> <P> Nicotine is the active chemical constituent in tobacco, which is available in many forms, including cigarettes, cigars, chewing tobacco, and smoking cessation aids such as nicotine patches, nicotine gum, and electronic cigarettes . Nicotine is used widely throughout the world for its stimulating and relaxing effects . Nicotine exerts its effects through the agonism of nicotinic acetylcholine receptor, resulting in multiple downstream effects such as increase in activity of dopaminergic neurons in the midbrain reward system, as well as the decreased expression of monoamine oxidase in the brain . Nicotine is addictive and dependence forming . The most common source of nicotine, tobacco's, overall harm to user and self score as determined by a multi-criteria decision analysis was determined at 3 percent below cocaine, and 13 percent above amphetamines, ranking 6th most harmful of the 20 drugs assessed . </P>

Substances which change the central nervous system include