The article used for the claim, Celsius Causes Brain Fog, goes over topics on how caffeinated drinks can lead to brain fog and fatigue.
The article, Caffeine & Brain Fog, starts by providing information on what brain fog is and what kind of correlation it may have with caffeine. Our author refers to brain fog “as hazy, unclear state of mind” and is most likely linked to chronic fatigue syndrome (CFS), which is fatigue that interferes with daily activities, which is said to affect about 1 million Americans. This is important because the author ties brain fog to caffeine and how the effects of caffeine affect how we feel throughout the day. Caffeinated drinks, such as coffee, are very quickly absorbed and distributed throughout your body, including to your brain. It's here that caffeine elicits its most classic effect — helping keep you alert and awake.” However, this may harm your sleeping schedule and ultimately lead to CFS and brain fog.
Additionally, this doesn't prove that Celsius or caffeinated drinks cause brain fog. In the article it says that “fatigue and brain fog should be easy to treat because they are likely related to your lifestyle. Changing your habits and routine can go a long way in giving you the energy you need to be your sharpest, most alert self who is ready to take on the day.” Indeed, caffeine could potentially lead to CFS due to its nature to keep you awake and ruin sleeping patterns and other healthy habits. However, the claim created a narrative that Celsius was the direct cause of brain fog, yet the brand was never included in the article. This clickbait headline provided no information on the side effects of Celsius or studies proving that Celsius has directly caused brain fog in the past. Although Celsius indeed contains caffeine, which is a substance that could potentially lead to brain fog, it is not the direct cause of brain fog. Instead, CFS and brain fog are the direct effects of unhealthy lifestyles and poor sleeping habits.
Reference:
https://www.houstonmethodist.org/blog/articles/2021/oct/caffeine-sleep-how-long-does-caffeine-keep-you-awake/