In drug cases, the police are often involved in the planning stages of the drug transaction, whether by using an undercover detective who poses as a buyer or seller, by using a confidential informant or by using surveillance to record the discussions between the parties involved in the drug transaction. With that kind of evidence, prosecutors often bring conspiracy charges in addition to the drug sale and/or purchase charges once the transaction has been completed.
However, police and prosecutors will sometimes assume the elements of a drug conspiracy are in place just because there were preliminary discussions about the drug transaction and the drug transaction took place. Conspiracy is a crime completely separate from the actual drug sale and purchase crimes. In order to prove a conspiracy, the state has to prove that there was an agreement between two or more people to commit the same offense. Therefore, if the state only has evidence that two people met at a certain location at the same time and completed a drug deal, they can assume there was an agreement to buy and sell drugs but without proof of the actual agreement, there is insufficient evidence to prove a conspiracy. In other words, proof of the drug deal, even where it appears to be elaborately planned, is not sufficient evidence to prove and agreement an a conspiracy.
Finally, where the state has evidence that two people agreed to a drug deal, it still may not be sufficient to prove a conspiracy. The proof of the agreement has to establish that the parties agreed to commit the same offense. So, if the state has recordings or other evidence that a buyer and a seller agreed to a drug transaction and actually went through with it, this is not a conspiracy because the two parties did not agree to the same offense. The two parties agreed to, and committed, separate offenses, i.e. sale of drugs and purchase of drugs. There is no common goal because one is selling and the other is buying and those are separate drug crimes.
Sometimes prosecutors like to add conspiracy charges in drug cases because it increases leverage for the prosecutor and adds exposure for a defendant. Prosecutors may look at evidence that the defendant(s) planned the drug deal and went through with it and assume they have a valid conspiracy charge. However, conspiracy is a completely separate crime and evidence of planning and/or a drug deal may not be sufficient to prove a conspiracy.