Everyone knows you need drug test documentation to get an original MMC and to renew it. Most people also know that if you are only adding STCW endorsements, you do not need to submit drug testing paperwork with the application.
Where people get hung up is upgrading. Some upgrades need a drug test and some do not, and it all depends on whether the Coast Guard considers what you are asking for a raise of grade or an increase in scope. A raise of grade needs a drug test and pays the higher evaluation fee. An increase in scope, which the Coast Guard also calls a modification or removal of limitation, needs no drug test and pays the lower fee.
The good news is you do not have to guess. Go to the National Maritime Center fees page at https://www.dco.uscg.mil/nmc/fees/ and click on "The RIG vs Modification Fee Tables." Find the credential you hold across the top, find the credential you are applying for down the side, and read the letter in the box. R means raise of grade, so a drug test is required. I means increase in scope, so no drug test is needed. That table is how the NMC makes the call, so checking it is how you know for sure.
The clear raises of grade are the steps up through the unlimited licenses. Third Mate to Second Mate is an R. Third Assistant Engineer to Second Assistant Engineer is an R. Drug test, higher fee, no surprise there.
What surprises people is how many upgrades are an increase in scope (I) and do not require drug test documentation. Adding oceans to a near coastal license is an I. So is adding near coastal to an inland license, or adding Great Lakes or Western Rivers. Adding Mate Pilot of Towing to a Master 200 GRT or a 500 GRT Mate is an I. Going from Master 100 GRT to Master 200 GRT is an I. On the engine side, a horsepower increase is an I, and so is adding steam, motor, or gas to a propulsion mode you already hold.
The same is true across most of the ratings. Moving between AB endorsements, say AB Special to AB Limited to AB Unlimited, is an I. Adding Lifeboatman to an AB or QMED is an I. Moving between QMED endorsements is an I. Tankerman Assistant to Tankerman PIC is an I. Tank Barge PIC to Tank Vessel PIC is an I. Adding DL or LG to a tankerman endorsement is an I. None of those need a drug test.
If you do land on an R, you have three ways to prove it. The most common for professional mariners is a letter on your employer's letterhead stating you have been in a random testing program for at least 60 of the last 185 days with no failed or refused test. You can also take a DOT five-panel drug test within 185 days of the day you submit, with the result signed by the Medical Review Officer. Or you can use a letter on your employer's letterhead stating they hold evidence of a passed pre-employment test within the last 185 days. As always, a letter from a union does not count.
If you check the table and still are not sure which box you land in, you can create a free account at mmseas.com to ask one of our USCG Licensing Experts for help. It is a quick answer, and a lot faster than finding out after you submit.