Hey man, it may be a little late for new advice, but watch out with promethazine.
I recently had an experience with this stuff called Tussionex, which I guess is a few levels higher than your codeine drink. Tussionex is hydrocodone + chlorpheniramine.
I've used opiates in the past and had never encountered any problem besides stomach acid. However, unlike just regular opiates, these opiate syrups contain a substance that intensifies the opiates interaction with your body.
The reason I call for caution when using these syrups is because the opiate intensifiers have an EXTREMELY long half life. I do not fully understand the science behind the interactions, I can only boldly state that those intensifiers become very very unpleasant once you exceed the recommended dosage.
Unlike many other drugs, it is in your best interest and health to follow the recommended dosages for the opiate 'potentiators' with an extremely long half life such as chlorpheniramine.
I am assuming chlorpheniramine and promethazine are similar and have similar side effects. Even if I am wrong, start small and make sure that the opiate potentiator has fully left your system before you redose. It should take about 1-2 days. Be smart.