LSD is a serotonin (5-HT) receptor agonist. This means that it binds to the serotonin receptors in your brain. Serotonin is a neurotransmitter primarily in control of the emotional/sensory systems of the brain, and it is closely associated with the dopamine (sort of a reward system) systems as well. LSD present in this system is the cause for the dialated pupils, of course. Acid does not physically harm your brain and is not neurotoxic (contrary to the "acid melts holes in your brain" theory). Basically, there is somewhat of a conscious "filter" in your normally functioning brain (serotonin) that limits the conscious activity and lets you clear your head and focus on one thing at a time. of course, while on acid, thoughts, ideas, and emotions float freely around in the head and this filter is effectively lost. This is what gives LSD the characteristics which let the user turn inwards and explore the consciousness and the makings of one's personality. It's presence in the 5-HT system is also the reason for the visuals and hallucinations, be it visual, auditory, tactile, etc. Now, LSD is just a collection of hydrogen, carbon, oxygen and nitrogen atoms. It doesn't carry a "trip" with it, nor does it contain instructions on how you are supposed to react to it. The entire trip is only aided by the LSD, in that it gives your brain the sensitivity and openness to elaborate on the most deep and personal aspects of the trip. The visuals are neat, but it gets so much better. The thoughts and emotions floating around in your head create a unique experience every time, and the depth in which emotions are experienced gives real potential to the psychotherapeutic aspect of the drug- occasionally, past trauma or current issues will be surfaced by the trip, causing a "bad trip." Contrary to popular belief, a bad trip is a psychologically healthy experience. Rather than the trauma that was there weighing you down constantly, it is brought forth and you are forced to deal with it- a brutal but often necessary process. Upon leaving the "bad trip," the issue is resolved, and you feel happier about it. LSD if taken in too large a dose, as you may very well know, can cause you to lose your grip on reality, think you're dying, being ripped apart by demons, etc. It's difficult to explain the power of this drug. However, such an experience is not an "overdose." Given the LD50 (dose at which 50% of the tested subjects die) of lsd, you'd need somewhere in the neighborhood of a million dollars worth of acid to overdose- so really, it's just some kind of mega psychological trauma and you'll emerge from the trip physically fine. However, LSD has been known to bring forth psychological disorders in those with a family history of such or those who are skirting around a disorder in the first place. LSD, in my opinion, is one of the safest drugs you can take, as long as you're prepared for the experience. The doses are so minuscule that were it to be adulterated with any other drugs, they wouldn't do anything, because there just isn't enough there to take effect. As well, the sheer improbability adds to the safety, as well as the fully non-physical action of the drug. The pharmacology leaves the experience almost entirely up to your mind and although the experience is very "pushy," it doesn't stimulate your brain to do anything.

I’d have to agree with most of the previous answer and wouldn’t have commented other than to emphasize that although LSD is likely the most benign drug regarding physical effects, emphasis has to be put to the potential of lasting psychological damage. An LSD experience can give you a marvelous memory of climbing inside of yourself and looking around, mental clarity on unlogic (the things you will never find in a science or math text), or just visiting places that you’ll never take the family to on an afternoon drive, but it can also result in a traumatic experience that can change your character as effectively as any violent event and can result in introversion, permanent paranoia and institutionalization. It can change you. The brain is under development until the mid teens. You are developing character traits well into your twenties. Experimentation with any drugs will alter these developments. LSD sounds like a nice safe evening of fun and discovery when you talk to people that have tried it and liked it but the reality is you don’t know how it is going to effect you and moreover...you don’t know what it is mixed with. Strychnine poisoning is sometimes associated with LSD tablets. Dosages are unknown. You might have twice as much on one blotter page as you do on another of the same type. While you cannot overdose on LSD (meaning there is no physical damage), too much can push your mind to places of no return. It is a purely psychological drug and that is where any potential damage will happen. Another negative aspect is visual contamination. Reports from long term frequent LSD users indicate that permanent patterns can develop in your sight. Imagine living out your life looking through a sheer checkerboard curtain. There is much in life that can be enjoyed straight. There is no high to compare with the perfect evening with the perfect love. Or an evening with a best friend just being goofy. If you want to go someplace in your mind a good book will do that. While I’ve enjoyed my escapades with LSD (but I know many that cannot say that) I maintain that they are by far not my best times. Be all you can be by avoiding drugs of any kind, at least until you know who you are and have a life plan in the works. Hindering your development will leave you wondering for the rest of your life who you could have been and what you might have accomplished.