Which route of administration involves entering the bloodstream through mucous membranes in the mouth?

Buccal administration is the correct answer because it specifically refers to the method of delivering substances through the mucous membranes in the mouth, particularly between the gums and the inner lining of the cheeks. This route allows for the absorption of the drug directly into the bloodstream, bypassing the digestive system.

This process can lead to a quicker onset of action compared to oral administration, where substances must first pass through the gastrointestinal tract before entering the bloodstream. Buccal administration is often used for medications that need fast absorption or those that would be less effective if ingested orally due to first-pass metabolism, which occurs when a drug is metabolized in the liver before it reaches systemic circulation.

In contrast, injection involves administering substances directly into the body using a syringe and needle, and the rectal route involves inserting substances into the rectum, both of which do not utilize the oral mucous membranes for absorption. Oral administration, while involving the mouth, refers to swallowing substances rather than having them absorbed through the mucous membranes.