A persuasive essay trying to convince people that some over the counter drugs should not be sold to just anyone without limits could contain any or all of the following arguments:
1) Some of those drugs are abused - such as cold medicine by teens, and some contain drugs that make people drowsy and too dangerous to drive. Age limits on who can buy those drugs (such as 18 or older) is reasonable
2) Sudafed is used by methamphetamine manufacturers as a basic ingredient, so placing limits on age or on quantity is reasonable
3) Some drugs are dangerous for women who are pregnant or may become pregnant, or that are taking other medicines too, so requiring that such drugs be asked for over the counter and getting a pharmacist consult when you buy them is a reasonable restriction, so they can refuse to sell them to those they think may be endangered by them
When organizing this essay, depending on how long or detailed your assignment is, you would want your first paragraph to be your main thesis argument. If you used the above material, you could state that "Limiting sales of over the counter drugs to certain people and age groups is reasonable because...", then you can list (not explain) specifics from the main points you wish to make that prove that statement true.
I don't know if you have CVS Pharmacies in your area, but they just made news recently on this subject. CVS had apparently been selling large amounts of cold and allergy medicines that includes pseudoephedrine, a methamphetamine ingredient used by many drug traffickers and illegal drug lab creators. CVS, the U. S.'s largest drug store chain, has agreed to pay a $75 million fine for their uncontrolled sales in California and Nevada. You might want to consider using this information as part of your persuasive essay. I've included a link that includes additional info.