Water-soluble drugs, such as the antibiotic tetracycline, are soluble in blood but are - in a (n) uncharged state because they are weak acids. Once - the cell, these drugs become -, associating with -, to become positively - and soluble in the cytoplasm.

Receptors are specific molecular components of a biological system with which drugs interact to produce changes in body function. This fundamental event of drug-receptor interaction initiates communication, through signal molecules (first messenger) of great structural and functional diversity, with receptors coupled to ionic channels, to G-proteins, catalytic or regulators of DNA transcription, as well as The integrated support of second messengers. It also mentions certain diseases that arise due to dysfunction of these receptors of the receptor-effector systems, such as the appearance of aberrant receptors or as products of "oncogenes".