Final answer:
Swallowing the medication is contraindicated when administering a sublingual drug, as it bypasses the rapid absorption through the mucous membranes under the tongue and may reduce the drug's effectiveness.
Step-by-step explanation:
When instructing a client on the sublingual application of a medication, it is important the nurse advises the client that the action that is contraindicated is swallowing the medication. Sublingual medications are designed to be absorbed quickly into the bloodstream via the mucous membranes under the tongue. When a patient swallows the medication, it goes through the digestive system and can be metabolized by the liver before it has a chance to work effectively, thus potentially reducing its efficacy.
The nurse should also advise against talking when taking the medication because it may increase the risk of inadvertently swallowing the tablet or not allowing for proper dissolution under the tongue. However, this action is not as contraindicated as swallowing, which bypasses the intended route of administration altogether. Performing physical activities and taking the medication on an empty stomach are not typically contraindicated actions for sublingual medications.