system
Given the following post, respond as if you're commenting on Reddit:
human
Why is LSD so powerful?

A normal LSD dose is about 100μg and it can cause hallucinations but the threshold is 25μg. 
How can such small quantities of this substance alter your mind?
This has been for a long time one of my curiosities. Thanks for the answers.
gpt
Not speaking to LSD specifically, but for a lot of drugs, a little bit goes a long way.  Drug X, for example, might result in anti-depressant effects by reducing seratonin reuptake, but that doesn't mean the Drug X is causing that *directly*.  Drug X might be a ligand for Enzyme Y, which causes increased levels of hormone Z to be produced, and hormone Z is what causes the increased seratonin levels.  Because of the signaling pathways, the amount of seratonin released might be very disproportional to the amount of Drug X ingested.  

To put that in other words, 1 molecule of drug X might cause 10 molecules of hormone Z to be released, and each molecule of hormone Z released causes 10 molecules of hormone Y (which is what is *actually* causing the effect of the drug) to be released.  Thus 1 molecule of drug X can lead to the release of 100 molecules of hormone Y.  This mechanism, extrapolated out to dozens or hundreds of steps, is how a very small amount of a drug can lead to very pronounced effects.