Hallucinogens are drugs that cause altered states of perception and feelings. They can produce unexpected flashbacks of altered states of mind. Some are natural substances such as mescaline and psilocybin that come from plants (cactus and mushrooms.  Others are manufactured such as LSD which is made from lysergic acid which is found in a fungus that grows on rye and other grains. 

LSD binds to and activates a specific receptor for the neurotransmitter serotonin, which normally binds to and activates its receptors and then is re-absorbed into the neuron that released it.  LSD binds very tightly to the serotonin receptor, causing a greater than normal activation of the receptor.

Because serotonin plays many important roles in the brain, when LSD activates its receptors users experience a widespread effects that include mood swings, altered perceptions, delusions, and visual hallucinations. 

MDMA (ecstasy) and PCP, while not a true hallucinogen, are often classified as this type because of they change the way how the brain perceives time, reality, and surrounding events.  Users can hear voices, see images, and feel sensations that do not exist.  PCP and MDMA are both physically addicting.

PCP interferes with the functioning of the neurotransmitter glutamate, which is found in neurons throughout the brain.  Like many other drugs, it also causes dopamine to be released from neurons into the synapse.  PCB can cause effects that mimic the primary symptoms of schizophrenia, such as delusions and mental turmoil.  People that use PCP for long periods of time have memory loss and speech difficulties.

MDMA, which is similar in structure to methamphetamine, causes serotonin and dopamine to be released from neurons in greater amounts than normal.  Once released, these can excessively activate these receptors.  MDMA can destroy serotonin-containing neurons causing hallucinations, confusion, depression, sleep problems, drug craving, severe anxiety, and paranoia.