What is a psychoactive effect?
A psychoactive effect is an affect on mood or behavior caused by the misuse of drugs.
Yes, heroin is a psychoactive drug. The DEA classifies it as a "Schedule-1" drug which also includes hallucinogenics as LSD, PCP, etc. Pharmacologically, heroin and methadone are both "pro-drugs" which metabolize to opiate-like molecules like morphine in the body. Note that the term "psychoactive" does not medically indicate a drug of an illegal or recreational nature. Many Schedule 2,3,4 drugs are also psychoactive in nature, and include morphine, Demerol, Xanax, codeine, barbiturates like Seconal, Talwin…
Excitatory psychoactive drugs such as nicotine and cocaine primarily affect the central nervous system. They increase post-synaptic transmissions and may result in addictions and substance abuse. These stimulants increase the alertness of the user by mimicking the action of neurotransmitters or delaying the breakdown of neurotransmitters. They can also affect the transmission of optical signals in the thalamus of the brain.
The major psychoactive chemical compound in cannabis is Δ9-tetrahydrocannabinol (commonly abbreviated as THC). However, there are at least 66 other cannabinoids present in cannabis, including cannabidiol (CBD), cannabinol (CBN) and tetrahydrocannabivarin (THCV) that contribute to the psychoactive effects.