What is the effects of a drug suddenly returning days after a person used the drug?

The effect is called a drug flashback. it is very common in users of LSD, a drug of abuse.

No definitive explanation is currently available for these experiences. Any attempt at explanation must reflect several observations: first, over 70 percent of LSD users claim never to have "flashed back"; second, the phenomenon does appear linked with LSD use, though a causal connection has not been established; and third, a higher proportion of psychiatric patients report flashbacks than other users. Several studies have tried to determine how likely a user of LSD, not suffering from known psychiatric conditions, is to experience flashbacks. The larger studies include Blumenfeld's in 1971 and Naditch and Fenwick's in 1977, which arrived at figures of 20% and 28%, respectively.