The primary selling point of Thriiv is that one can experience enhanced mental performance without any downside. A quick example is coffee will give you energy for about 5 hours then come with a huge crash making one lethargic for the following 2 hours. Thriiv has no crash, and for most users, has no noticeable side effects.
At the same time there are inevitably going to be people who do experience side effects. Most commonly the side effects are dry mouth, dehydration, headache, or brain fog. Here is an article explaining more in detail the common side effects of thriiv and why some of them are actually beneficial.