Some drugs (e.g., marijuana and heroin) have effects similar to neurotransmitters (chemical messengers) which are naturally produced by the brain. Other drugs, such as cocaine or methamphetamine, can cause the nerve cells to release large amounts of natural neurotransmitters mainly dopamine or to prevent the regeneration of these natural neurotransmitters, which is used to stop the signalling between the neurons. Dopamine is responsible for controlling movement, motivation, emotion and feelings. The release of the large amount of dopamine produces euphoric effects (such as relaxation, anxiety relief, stress relief, mood lift etc.). The effect of this drug or the feeling after taking this substance sets a signal in the brain that makes people want to repeat the rewarding behaviour of the drugs. If the person continues to use the drugs, the brain adapts to the large amount of dopamine and tries to overcome the overwhelming effect by producing less dopamine or by reducing the number of dopamine receptors in the brain. This results in the effect of dopamine being reduced in the reward circuit, and this reduces the person abusing the drugs ability to enjoy the drug and the effect that was previously derived from the drug. This makes the person to continue abusing the drug so as to get the effect of dopamine back but large amount of the drug will be needed to achieve the same previous effect of dopamine and this is known as the tolerance effect. Long-term abuse causes changes in other chemical systems of the brain and reward circuits also. Glutamate is a neurotransmitter that influences the reward circuit of a personâ€™s ability to learn. When the maximum concentration of glutamate is reached by drug abuse, the brain attempts to change this, which can damage the mental function of the abuser. This can lead to changes in areas of the brain that are design for judgment making , decision making, learning, memory control and behaviour control. These changes can make the abuser continue taking the drugs because of the psychological effects despite the adverse effects the abuser is having.