The two options to be compared in the trial are firstly, a once daily preventive dose (prophylaxis) of an antibiotic routinely used for this purpose (either nitrofurantoin or trimethoprim or cefalexin), and secondly no prophylaxis. The investigators think that an overall decrease of 20% or more in the frequency of UTI would be large enough for future patients using CISC who get troublesome recurrent UTIs to be offered antibiotic prophylaxis routinely. The investigators will also assess any harm caused by continuous use of antibiotics, particularly side effects for those people taking them and changes in the resistance of bacteria to these antibiotics. The investigators can then work out whether the balance between the benefits and harms make the use of prophylaxis worthwhile to people carrying out CISC and for the NHS as a whole.