Holding in your pee for a long period of time can often stretch your bladder and in some rare cases also extend the external sphincter muscles [these muscles are the gate-keepers of the pee point of your bladder].
There is an automatic feedback mechanism in the bladder which sends a signal to your brain when it’s full, which then urges you to get to the nearest toilet but if you keep yourself from peeing often, your body might lose the ability to know when it’s time to go pee.
You might also risk urine leakage when your bladder is full & the case of not emptying it all the way. Not only can this increase your need to urinate more often since your bladder fills up quicker, it can also lead to a serious disorder called Urinary Retention.
Stretching of bladder and not realising when to go are not the only adverse effects; the longer you hold your urine the possibility of the bladder becoming a breeding ground for bacteria increases. This bacteria can lead to Urinary Tract Infections (UTI), which can spread to kidneys and cause another level damage to the body.
Holding in your pee often and not drinking enough liquid cause kidney stones to develop. Because of it you may experience a persistent ache in your lower back, nausea, and pain when you pee.
Pressure on your knees because of all the hovering that you might do over a public toilet seat and an actual possibility of contracting UTIs are few of the many shortcomings of using an unhygienic toilet.