Retrievanol, colloquially known as Flash, is a memory drug.
Retrievanol was developed by Gautuma Medical as a drug for Alzheimer's patients and administered only under strict protocols. However, the secrets to Flash became public knowledge and became popular as a recreational drug among "people looking to escape their lives" when Alzheimer's was cured.
Sonya Valentine seemed to have manufactured her own supply of the drugs in the present to trade with the local gangs for their services.
When taken, through eye drops, Flash gives the user a vivid recollection of a memory you choose to remember with the drug in your system. According to Ricky, the event you see does not even have to be real; the user can still experience it in a flash as long as they can imagine it as if it did happen. Also, apparently according to Nadia, a re-experienced memory through Flash, although extremely detailed, may not be completely accurate, as her experience was never as "perfect" as it was through Flash, and that what the user sees through Flash might just be "what you hope is true".