A proportion of uncomplicated bladder infections do resolve without antibiotics, helped by drinking plenty of water and over-the-counter pain relief for discomfort. However, it is not always possible to predict which will settle and which will worsen, and an untreated infection can ascend to the kidneys, which is more serious.
Antibiotics remain the standard treatment for confirmed or troublesome UTIs because they shorten symptoms and reduce complications. The decision to wait or treat should ideally involve a clinician or pharmacist, who can assess your symptoms and risk factors.
Do not delay care if you are pregnant, have diabetes, are male, have recurrent infections, have a catheter, or develop fever, back pain, or vomiting, as these situations need prompt treatment. If you self-manage a mild case and it does not improve within a day or two, see a doctor.