The myth of acid flashbacks is rooted in anecdotal evidence and stories from the 60s and 70s, but there is no concrete evidence to support its existence. The minimum dose of LSD is a quarter microgram, much smaller than a typical acid hit today.
The lethal dose of LSD (LD50) has never been determined; however, it is estimated to be about 70,000 times the minimum dose, and there have been cases of heart attacks and strokes associated with LSD use but it's unclear if the drug was the direct cause.
Ketamine is a relatively uncommon drug and doesn't cause any physical or psychological dependence. Its non-addictive properties set it apart from other substances, making it useful for medical purposes.