Is it ok to take just one antibiotic a day?

Full Amount. It is best to take your antibiotics exactly as your physician prescribes. So take the full amount a day, and for the full length of time. If you miss a dose, try to take it as soon as you remember without taking multiple doses at one time. And it is important that even if you're feeling better after a few days-still take the full prescription to make sure you're rid of the infection.
Antibiotics. Antibiotics should only be used if prescribed for the right reasons usually for bacterial infections. The dosing is dependent on the half life of the ingredients, ie. Many antibiotics like Azithromycin will stay to act for 24 hrs, however others like Amoxicillin will act for 8- 10 hrs and hence have to be taken 2 or 3 times per day. Also antibiotics are given usually for a short duration like 10d.