These substances are metabolized by the liver. Stimulants elevate the mood, depressants slower down the activity of the brain, and body. Once they are taken inside the body, they reach the specific target organs, act at the site, and the debris is metabolized by the liver and thrown out.
Yes, if people have addiction history in their lives or their family history they should not be able to purchase large amounts of these products. Tobacco should have more restrictions against 18 year olds, even though they cannot legally purchase it many still do.
The customer is probably having an allergic reaction to something on the seafood platter.