An over the counter (OTC) drug is one that can be sold with out a prescription.
The OTC drug market has high volumes, but the drugs are much cheaper. OTC drugs are not designed for the treatment of serious conditions. This limits the advantages any in-patent drugs can offer over generics. Brands are more important than in the prescription drug market, but other aspects of marketing are easier — for example, the need for large sales forces (that sell prescription drugs to prescribers) is reduced.
OTC drugs make up only a small proportion of the sales of the major pharmaceutical companies, but they are more important for some of the smaller specialist pharmaceutical companies. Some are manufactured by companies outside the sector — such as Reckitt Benckiser.