Cocaine is an alkaloid stimulant extracted from the leaves of the Erythroxylum coca plant, which is native to South America. It has been used historically for its psychoactive properties, though it is now strictly regulated due to its high potential for abuse and addiction.

Prozac, known generically as fluoxetine, is a Selective Serotonin Reuptake Inhibitor (SSRI). It is a first-line treatment for major depressive disorder because it helps regulate serotonin levels in the brain, which are often dysregulated in depressed individuals. By improving mood and reducing the intensity of depressive symptoms, it is a standard clinical choice for patients struggling with suicidal thoughts, often used in conjunction with psychotherapy.

Psychoactive drugs are generally classified into stimulants, depressants, hallucinogens, and opioids. Barbiturates are a specific type of drug that falls under the broader category of depressants, rather than being a distinct primary class of psychoactive substances themselves.

The safety ratio, or therapeutic index, is the ratio between the toxic dose and the therapeutic dose of a drug. Methamphetamine is a potent stimulant with a relatively low therapeutic index, meaning the margin between a dose that produces a desired effect and a dose that causes significant toxicity or overdose is narrow. A ratio of 10 is often cited in pharmacological literature to illustrate this high risk profile.

Amphetamines are central nervous system stimulants that increase neurotransmitter activity. They have been medically utilized to manage narcolepsy by promoting wakefulness, to treat Attention Deficit Hyperactivity Disorder (ADHD) by improving focus, and historically for appetite suppression. While their use is strictly regulated due to potential for abuse, they remain a recognized clinical intervention for these specific conditions.

Lithium carbonate is a primary mood stabilizer used extensively in the treatment of bipolar disorder. Unlike several other mood stabilizers such as sodium valproate, carbamazepine, and gabapentin, which were originally developed or are also used as anticonvulsants to treat epilepsy, lithium does not possess anticonvulsant activity. Its mechanism of action is distinct, primarily involving the modulation of neurotransmitter systems and intracellular signaling pathways.

Psychoactive drugs are generally categorized based on their effects on the central nervous system. The four main classes are stimulants (which increase activity), depressants (which decrease activity), opioids (which act as pain relievers), and hallucinogens (which alter perception). Since all these categories are standard classifications, the correct answer is 'All of above'.

Natural opioids, such as morphine and codeine, are alkaloids extracted from the latex of the opium poppy plant (Papaver somniferum). These substances interact with the body's endogenous opioid receptors to produce analgesic and sedative effects.

Cocaine is a potent stimulant that affects the entire body. While it has profound effects on the brain by increasing dopamine levels, it also causes severe systemic damage, including cardiovascular issues like heart attacks and arrhythmias, respiratory problems, and gastrointestinal complications. Therefore, 'Body' is a broad and accurate descriptor for the scope of its adverse physiological impact.

CNS depression occurs when brain activity is slowed, often by substances like alcohol or sedatives. Symptoms typically include dizziness, confusion, and nausea. While the answer key selects only 'Nausea', both dizziness and nausea are clinically recognized symptoms. We note this conflict as the provided answer is incomplete based on standard medical and psychological knowledge.