The short version
- The doctor threshold is international, not American: 100 or more persons on an international voyage of more than three days[1].
- That threshold comes from MLC 2006 Standard A4.1.4(b), and the United States has not ratified the MLC[2].
- The Coast Guard has said it “will not mandate enforcement of its requirements on U.S. vessels”[2]. There is no US rule that puts a doctor on a ship.
- A US-flag vessel can still be detained in a foreign port for failing the standard, through the no more favourable treatment clause[2].
- Below the threshold, MLC A4.1.4(c) requires a seafarer in charge of medical care, or a seafarer competent in medical first aid, trained to STCW[1][7].
- What US law does require is a medicine chest on a foreign voyage[4] and, on some hulls, a hospital space[5].
Google answers this question without mentioning the United States once
Search the term and the AI Overview returns the MLC threshold, the ILO convention behind it and a UK flag-state note. Page one carries a Facebook post, a Quora thread and a Reddit thread inside the top eight. There is no US source and no US answer anywhere on it.
For an operator of a US-flag vessel that is not a small omission. It is the whole answer, because the instrument everyone is quoting does not bind the United States.
US Coast Guard, MLC 2006 policy
“The U.S. has not ratified the MLC.” And: “Until such time that the U.S. ratifies the MLC, the Coast Guard will not mandate enforcement of its requirements on U.S. vessels or upon foreign vessels while in the Navigable Waters of the United States.”[2]
The threshold everyone quotes, and where it comes from
MLC 2006 Standard A4.1.4(b) requires that ships carrying 100 or more persons and ordinarily engaged on international voyages of more than three days’ duration shall carry a qualified medical doctor[1]. Persons, not passengers, so crew count toward it. Flag states may set stricter rules, and several do[3].
Below that line, paragraph 4(c) requires either at least one seafarer in charge of medical care and administering medicine as part of regular duties, or at least one seafarer competent to provide medical first aid, with training meeting STCW[1][7].
| Vessel | What the international standard asks | What US law actually requires |
|---|---|---|
| 100+ persons, international voyage over 3 days | Qualified medical doctor aboard[1] | Nothing. The US has not ratified the MLC[2] |
| Below the threshold | Seafarer in charge of medical care, or medical first aid, trained to STCW[1] | No equivalent duty in 46 CFR for most vessel classes |
| Foreign voyage, any US vessel | Medicine chest under flag law | 46 U.S.C. 11102 requires a medicine chest, with no tonnage floor[4] |
| 12 or more crew, voyage over 3 days | Not addressed directly | Hospital space under 46 CFR 92.20[5], and 108.209 for MODUs[6] |
| US fishing vessel | Not covered by the MLC in practice | 46 CFR part 28 sets first aid and chest duties, no doctor, no crew fitness certificate[9] |
The trap: no US duty, foreign detention anyway
The Coast Guard document that states the US has not ratified the convention also states the consequence in the same breath. US vessels not in compliance with the MLC “may be at risk for Port State Control actions including detention when operating in a port of a ratifying nation”[2].
That is the no more favourable treatment clause working against a non-ratifying flag. The vessel is compliant at home and exposed abroad. An operator reading only 46 CFR will never see it coming, and an operator reading only the MLC will over-build for domestic trade.
The practical test is the trade, not the flag. A US-flag hull that never leaves domestic waters carries no MLC exposure. The same hull on a foreign fixture inherits the whole standard at the first ratifying port.
What a ship without a doctor actually needs
Most commercial vessels never reach the 100 person threshold, so the real question is what replaces the doctor. Four things, and only one of them is a person.
- A trained person. Under the international standard, a seafarer in charge of medical care or competent in medical first aid, trained to STCW[1][7]. US law does not impose this outside specific vessel classes, which is precisely why it gets skipped.
- A chest that matches the voyage. 46 U.S.C. 11102 requires a medicine chest on a foreign voyage and names no contents anywhere in the CFR[4]. Scope it against your flag state and the WHO International Medical Guide for Ships[8].
- A lawful route for scheduled medication. The moment the chest holds a controlled substance, 21 CFR 1301.25 governs who may dispense it[12], and a prescription cannot be used to stock it.
- A physician reachable from sea. The trained seafarer is the hands. The clinical decision, including whether to evacuate, needs a doctor who knows what is aboard.
Three standards, three different axes
Operators assume these instruments stack. They do not. Each one measures a different thing, which is why a vessel can pass one and fail another on the same voyage.
| Instrument | What it measures | Trigger | Binds a US-flag vessel? |
|---|---|---|---|
| MLC 2006 Standard A4.1 | Headcount and voyage length | 100+ persons, international voyage over 3 days[1] | No. Not ratified[2]. Enforced against it abroad[2] |
| EU Directive 92/29/EEC | Vessel category and voyage area | Category A, B or C chest[11] | Only through EU ports and EU charterers |
| 46 CFR 92.20 | Crew size and voyage duration | 12 or more crew, voyages over 3 days[5] | Yes. This is the one that actually binds |
| 46 U.S.C. 11102 | Voyage type only | Any foreign voyage, no tonnage floor[4] | Yes, and it names no contents[4] |
| STCW | Individual competence | Medical first aid and medical care certificates[7] | Through US STCW implementation for credentialed mariners |
Read the right-hand column and the practical answer appears. For a US-flag operator the binding rules are the hospital space trigger and the medicine chest statute, neither of which mentions a doctor. The doctor question only becomes real at a foreign port, and then it arrives as a detention risk rather than as a rule.
Where the European rule differs, and why it matters to US operators
Council Directive 92/29/EEC scales medical provision by vessel category rather than by headcount, and binds EU-flag vessels and EU ports[11]. A US operator trading into Europe meets a second standard built on a different axis, which we set out in the EU medicine chest categories.
The lesson repeats across every flag question on this site. The threshold that applies is the one attached to the voyage, and it is rarely the one attached to the hull. The same reasoning drives what changes on a change of flag and what reaches a foreign flag vessel in a US port.
Deciding this for a real fleet
The question is never “do we need a doctor”. It is “which standard attaches to each vessel on each trade, and what is the cheapest lawful way to meet it”. That is a mapping exercise across flag, route, headcount and voyage length, and it is what maritime medical consulting does. The chest side is MedChest and the scheduled medication side is controlled substances management.
Start with the six decisions, or have a physician read the fleet in a compliance audit. Speak to the team.
Common questions
When is a ship legally required to carry a doctor?
Under MLC 2006 Standard A4.1.4(b), when it carries 100 or more persons and is ordinarily engaged on international voyages of more than three days[1]. Persons includes crew, not only passengers. Individual flag states may go further[3]. There is no equivalent requirement in US federal law, because the United States has not ratified the MLC[2].
Does a US-flag vessel need a ship’s doctor?
No US regulation requires one. The Coast Guard has stated it will not mandate enforcement of MLC requirements on US vessels[2]. What US law does require is a medicine chest on a foreign voyage under 46 U.S.C. 11102[4], and a hospital space on vessels meeting the 46 CFR 92.20 trigger[5].
Can a US vessel be detained abroad for not carrying a doctor?
Yes, and this is the part most operators miss. The same Coast Guard policy states that US vessels not in compliance with the MLC “may be at risk for Port State Control actions including detention when operating in a port of a ratifying nation”[2]. A non-ratifying flag receives no more favourable treatment than a ratifying one. Domestic compliance does not travel.
What is required if there is no doctor on board?
Under MLC A4.1.4(c), either at least one seafarer in charge of medical care and administering medicine as part of regular duties, or at least one seafarer competent to provide medical first aid, trained to the standard of STCW[1][7]. In practice that person also needs a chest scoped to the voyage[8] and a record of what was given.
Do passengers count toward the 100 person threshold?
The standard says persons, which takes in crew and passengers together[1]. That matters for passenger vessels sitting near the line and for research vessels carrying scientific personnel, where a berth count can move a hull across the threshold mid-season.
Is a mariner medical certificate the same thing?
No, and the two are often confused. A medical certificate under 46 CFR part 10 subpart C is about whether an individual mariner is fit to hold a credential[10]. A ship’s doctor requirement is about what care capability the vessel carries. One is a person’s credential, the other is a vessel’s equipment and staffing question.
Which rule applies if we change trade mid-year?
The one attached to the voyage. A hull in domestic trade carries no MLC exposure, and the same hull on a foreign fixture inherits the standard at the first ratifying port[2]. We cover the sequence in changing flag state and the voyage-length logic in remote vessel medical planning.
