The U.S. Coast Guard (USCG) determines medical and physical standards for mariners. Credentialed mariners (Merchant Mariners) must obtain USCG Medical Certification as proof of meeting standards. Physical examination and certification are designed to ensure that mariners:
- are of sound health
- have no physical limitations that would hinder or prevent performance of duties
- are free from any medical conditions that pose a risk of sudden incapacitation, which would affect operating or working on
Drug and alcohol testing is also an important aspect of the mariner credentialing process, and drug tests are required initially and periodically as well as after certain incidents (such as marine casualties).