Do not act as a Part 107 crewmember on cold medicine that impairs you. Section 107.27 incorporates §91.17. Section 91.17(a)(3) prohibits acting as a crewmember while using a drug that affects your faculties in any way contrary to safety. A legal over-the-counter antihistamine that causes drowsiness is that kind of drug. "Only illegal drugs" and "it is legally sold, so I can fly" are both wrong.
The impairing-drug rule
Section 107.27 names the person manipulating the controls, the remote pilot in command, and the visual observer. All three must comply with §91.17 and §91.19. The impairment sentence is §91.17(a)(3). It does not ask whether a pharmacy sold the drug legally. It asks whether the drug affects faculties contrary to safety. Drowsiness from a cold tablet is an effect contrary to safety on a flight that depends on seeing the aircraft, seeing other traffic, and deciding when to land. If the medication is doing that, you do not act as a crewmember while it does.
Section 91.19 is a different rule. It concerns carriage of specified substances. It is not the source of the impairment prohibition, and it does not become the answer to a drowsiness question. The alcohol limits in the rest of §91.17, the 8 hours and the 0.04 concentration, are also different sentences. They are the alcohol rules page and the blood alcohol page. A pilot who skipped the beer and took a drowsy antihistamine has not found a loophole in the alcohol clock. The pilot has a drug-impairment problem under §91.17(a)(3).
Read the label and believe the warning. If the box says the medicine may cause drowsiness, and you feel that drowsiness, you are on notice. Section 107.17 points the same direction from the fitness side: do not manipulate the controls or act as remote pilot, visual observer, or direct participant if you know or have reason to know of a physical or mental condition that would interfere with safe operation. The medication item is specifically the §91.17(a)(3) prohibition. The fitness section is why "I notice I am impaired" is already enough to stop, even before you argue about the brand name.
A spring inspection is the worked case. The remote pilot takes a legal antihistamine at breakfast. By the site, the pilot is slow and heavy-eyed. The aircraft is registered, the airspace is Class G, and the client is waiting. None of that answers §91.17(a)(3). The pilot does not act as remote pilot or as the person on the controls while the drug affects faculties contrary to safety. Another current remote pilot flies the job, or the job waits until the effect is gone. The IMSAFE check is where medication is supposed to surface before the drive to the site.
Dehydration is a fitness problem, not a drug charge
Early signs of dehydration a remote pilot should watch for are headache, fatigue, dizziness, and reduced judgment. Improved concentration and increased strength are the wrong picture. The course list also includes cramps among the early signs. The prevention is ordinary and specific: drink water, take shade, and rest. Left alone, dehydration can progress to heatstroke, which is a medical emergency. This is not an alcohol-concentration question and it is not §91.17(a)(3), unless a drug is what caused the symptoms. It is a condition you can know about, which is §107.17.
Three hours into a hot survey, a headache and a foggy head are the early signs. The remote pilot stops, drinks water, and rests in the shade before judgment degrades further. Launching anyway, because the battery is full and the light is good, is flying with a condition that interferes with safety. The trap is to treat dehydration as discomfort. The tested signs include reduced judgment. A pilot who is only "a little thirsty" and already choosing poorly has the symptom the item is written to catch.
Hyperventilation: slow the breathing
Hyperventilation is over-breathing, often from stress or anxiety. It can cause lightheadedness, tingling, and visual problems. The appropriate response is to consciously slow the breathing rate. Holding the breath for several minutes is the wrong response. Breathing faster and deeper continues the over-breathing that caused the problem. The fix is slower breathing, not a bigger version of the same pattern.
On a windy rooftop, a remote pilot starts breathing fast, feels tingling in the fingers, and the picture of the aircraft goes soft. The performance step is to slow the breathing on purpose, and to keep the aircraft in a safe place while that happens, including landing if the pilot cannot see and think clearly. Speeding the breath up because it feels like more air is the trap answer. Clamping the breath for several minutes is the other trap. Neither one is the correction the knowledge test wants, and neither one is a safe way to finish the shot.
Medication, dehydration, and hyperventilation fail a flight for different reasons. The cold tablet is §91.17(a)(3) through §107.27. The headache and lost judgment in the heat are dehydration under the §107.17 fitness duty. The tingling and fast breath are hyperventilation, corrected by slowing the rate. Do not file all three under "illegal drugs," and do not fly any of them because the medicine was sold at a counter.
Practice the three items on the free practice test. The physiology lessons are at pricing.