Image from Bernd Helfert
Ad-drugs (also known as Play and Pay or Pay and Play depending on the type) are generally provided by various commercial enterprises as a form of incentive advertising.
In the case of the first class of drugs (Play and Pay) the user partakes of a sample of the drug and after receiving the primary effect proceeds to experience side effects that make it more likely that they will purchase the product that the seller of the drug is marketing.
The sophistication of ad-drugs varies widely. At the most basic level, a user may experience a strong desire to eat or drink either while under the influence of the drug or immediately thereafter, resulting in increased purchasing of consumable items including (so the provider hopes) the foodstuff being marketed by the provider. More sophisticated ad-drugs may cause the user to feel an urge for a particular type of food (which the provider happens to sell). At the most sophisticated levels, ad-drugs designed by Superior or even transapient minds may cause the user to be particularly receptive to noticing certain advertising symbols or signs (even neuro-fractal patterns) that the provider employs, or may even induce a craving in the user for not just a particular type of product but specifically the type and brand of product sold by the provider of the drug.
The other major class of ad-drugs, known colloquially as 'Pay and Play' (or sometimes 'Gift with Purchase' or 'Party Favor') are much simpler in their basic application. The sophont who wishes to partake of the drug must first purchase some item (or sometimes some number of items) from the provider in order to receive the drug as a free additional incentive or reward for doing business with the provider in questions. Although much simpler in basic concept than their 'Play and Pay' counterparts, this class of ad-drug is not noticeably more (or less) popular than the other. Both types of drug have found a niche in societies that permit them and both continue to thrive.
Ad-drugs come in a variety of forms, from sophisticated chemical or biological constructs, to software downloads, to transapient designed replicating swarms with limited replication scales but wide cross-species compatibility. They are found across the Civilized Galaxy and beyond with local usage or accessibility determined by a combination of local social ethic, ruling memetic, and the degree to which the local governing systems regulate (or don't regulate) the use of mind-altering substance with short effect windows and a guaranteed lack of unexpected side effects (When provided by licensed and reputable manufacturers and backed by transapient class consumer protection seals. Under all other circumstances, use is at the consumers own risk).