The mental effects that are associated with LSD are usually hallucinations or visual images that sometimes relate with physiology or religious aspects.
When the mental effects set in, the illusions seen are often very powerful and overwhelming, making the user of the drug want to stop. At lower quantities of LSD intense flashes of light can be seen. The lights seen are often very bright and in distorted geometric designs. Sometimes the hallucinations can be dream-like or free-flowing.
The mental effects of LSD seem harmless, but they can be very dangerous. When the hallucinations set in, panic reactions can also happen. The effects cannot be controlled, as the only thing that can stop them is time. It can also cause anxiety and other pyschotic reactions.
A diagram that shows what you see when you take LSD.