This can be difficult as there are many causes, some of which are not concerning and easy to fix like low fuel and some of which can be deadly, such as hyponatremia.

Consider fueling and hydration

Both overhydration/hyponatremia and dehydration can cause vomiting.
Have you been “drinking to thirst” or drinking substantially more than this? If you have been drinking too much and are dizzy you may be overhydrated and hyponatremic. NOTE: you can be hyponetremic even if you are using electrolyte drink and not just plain water.  Look for other signs such as swelling in the hands or legs or weight gain (these are not always present). If you have been drinking too much and are dizzy without other symptoms such as confusion, you should stop and rest. While stopped don’t drink more water but you can drink a concentrated sodium solution such as 4 bullion cubes in 125mL water (you may want to flavor this as well). You are usually okay to continue racing if symptoms resolve and you keep a close eye on your hydration status.

Dehydration
Are you dehydrated? Have you urinated recently and how did it look? If the answer is no or the urine looked very dark and you have not been drinking much you are likely dehydrated. Consider slowing or resting and increasing hydration. Does your urine look brown like soda or tea? If so you may have rhabdomyolysis and you should hydrate and seek medical attention.

NOTE: RECEIVING INTRAVENOUS (IV) FLUIDS IS EXTREMELY DANGEROUS WITHOUT CONFIRMATORY TESTING, LAB WORK, ETC. IN THIS SETTING. AVOID IV’S BOTH DURING AND AFTER A RACE FOR PRESUMED DEHYDRATION.

Fueling
Low fuel, i.e. blood sugar can cause dizziness. Eat something with quick sugar such as a gel. If improved you can usually continue but keep a focus on fueling

Consider the conditions and environment

Heat
Dizziness while running in heat may indicate heat related illness.

Overheating
(See Overheating)

Heat exhaustion
(See Overheating)

Altitude
High altitude is typically considered to begin at 8,000 feet although you may feel some effects at lower elevations. Most of the dangerous complications of altitude occur at over 10-12,000 feet. If you are racing at altitude, regardless of acclimatization you are at risk. You are at increased risk without acclimatization. Isolated dizziness at altitude may be a warning sign of developing altitude illness but is not usually dangerous in isolation. Decreased effort level and monitor for other symptoms. Dizziness with other symptoms, such as headache or poor coordination, at altitude are concerning. Descend immediately and seek medical attention. You may be at risk for severe complications of altitude such as High Altitude Cerebral Edema. Dexamethasone (a prescription steroid) is part of treatment. Of note it is on the WADA banned list so should not be taken as preventative treatment in a sanctioned race.

Dizzy with other symptoms

With vomiting
(see Nausea and Vomiting)

With headache
Consider conditions, hydration and fueling to further evaluate.

With chest pain
Stop, decrease exertion and seek medical attention.

With syncope (passing out)
– If this occurs during exercise (i.e. while you are running) you should stop and seek medical attention.
– If this occurs immediately after exercise, it is not necessarily concerning and may be a result of decrease venous return (blood flow) once the muscles have stopped moving.

Treatment
Rest, find shade, elevate the legs and monitor for other symptoms such as chest pain or vomiting. If dizziness is not improving seek medical attention.

With shortness of breath
This may be related to too high effort for the conditions or your fitness. Slow down but if symptoms persist or occur while at rest you should seek medical attention.