The practitioner, the patient, the resident, and the pharmaceutical "gift".
D004345:Drug Industry; D004470:Economics, Medical / Q000331:legislation & jurisprudence / Q000639:trends; D004501:Education, Medical / Q000191:economics / Q000639:trends; D006801:Humans; D010818:Practice Patterns, Physicians' / Q000191:economics / Q000639:trends; D014481:United States