From the LSD FAQ, available (along with a bunch of other good drug stuff) at

Addiction Potential
Zero physical addiction potential. Not something that makes you want to do it again immediately. Essentially zero psychological addiction potential.

Because of its extreme cheapness and potency, the purity of LSD in blotter form is not an issue: either it's lsd or untreated paper. The purity of powders, pills, and liquids cannot be assumed as safe....If there are 200 micrograms of LSD in a tablet, there could only be 200 mics of impurities present even if the LSD was originally only 50% pure (assuming nothing else has been added), and few compounds will produce a significant effect until a hundred to a thousand times this amount has been ingested.

Flashbacks (LSD) is easily water soluble, thus cannot form bodily deposits. Furthermore, it is metabolized and excreted in hours. The bogus "loosened crystal" description in not necessary to explain flashbacks, which are psychological phenomena....

....LSD acts to preferentially inhibit serotonergic cell firing and seems to spare postsynaptic serotnergic receptors. This preference is shared by other similar hallucinogens but in a limited fashion....

Drug Tests
No risk. Its not looked for, hard to find, and transient.

Acquired rapidly, within 3 days. Tolerance dissipates equally rapidly, without withdrawal, craving, or symptoms of addiction. Cross-tolerance can and is developed between other indole hallucinogens, eg, DMT and Psilocybin.