Hydrocodone cough syrup won't let me urinate, even after drinking 2 bottles of water - what can I do?

Stop it. All opiates or narcotics can cause urine retention. Best solution is to not take them. If you develop a painfully swollen bladder and cannot void, you may have to go to the er and have a catheter placed. In most folks, simply stopping the medication will fix the problem, fortunately.