During the trial, you may suffer from mild symptoms like headache or nausea. That may be caused by the medication. It might also be caused by needing to fast for the trial, for example. Those side effects are no fun, but fortunately are not dangerous.
Before a medication is tested on people, it has already been studied extensively in a lab. This enables us to predict well which side effects you might get and we can prepare you for them. It almost never happens that participants suffer from unexpected side effects. For example, an allergic reaction to the medication occurs on average once a year.