Antibiotic means ‘opposing life’. That's exactly what you are doing if you attempt to complete an athletic event while taking them.
Antibiotics attacks a range of bacteria living in your body. Some will be the bad guys causing you to express sub-optimal health. But you lose some of the good guys too. Your biology becomes a battlefield while things sort themselves out.
Runners lose pace on antibiotics. Disruption of gut balance under stress can trigger nausea, diarrhoea and vomiting. Certain kinds of antibiotics increase susceptibility to sunburn, dehydration and heat stroke. Others have been shown to increase the risk of tendon failure.
If you wanted to hobble a runner without them noticing antibiotics would be a great choice.
Still feel like ignoring the side effects? Think you are well enough to compete? That means you may have taken antibiotics unnecessarily in the first place. If so you made a previous mistake before the additional one you are now planning to pile on top of it.
Antibiotics are great for bacterial infection that don't respond to other treatment. That treatment should include resting, optimising diet and prioritising care of the affected parts of the body.
Antibiotics are not so great if you are taking them because you can't be bothered to do any of that properly. Unfortunately this is the most common approach people in our society take to sub-optimum health. We often take drugs to avoid slowing down and paying proper attention to our bodies’ needs. This is especially foolish for those attempting to train our bodies to a higher level of performance.
Spending arduous hours working on your body? Claim you are in sync with its requirements? Then why ignore and override it?