LSD (lysergic acid diethylamide) has many dangers. Unlike many other illegal drugs it is not addictive, but it is very powerful.
First, it is illegal. Possessing, buying or selling it carries the risk of a jail term. Moreover, it involves associating with criminals, who can be quite dangerous people.
Next, there is no quality control for illegal street drugs. You don' t know what you are actually getting, or whether the drug has been "cut" with dangerous chemicals.
Even with a chemically pure dose, the sheer power of the hallucinations makes it dangerous. You might decide you have wings and jump out of a building or something equally hazardous and potentially lethal. Some people become suicidal under its influence and a small percentage may have psychotic episodes.