How long after taking 50mg of Benadryl (diphenhydramine) do I have to wait before taking promethazine?

Not recommended . It is not a good idea to combine these two sedating first generationantihistamines half life of Benadryl (diphenhydramine) is 8 hours, that means 25 mgm is still in your system.
Potential drug … Interaction: combo can increase risk of sedation (cns depression), psychomotor impairment, dry mouth, blurred vision, difficulty urinating, constipation (anticholinergic effects). Why r u using both meds? Need more info to advise further.
Some drug overlap. There's a certain amount of overlap between the effects of these two drugs, which raises the question of how badly you need the promethazine. If it's prescribed to use only as needed, perhaps you will not need it for at least several hours after the benadryl (diphenhydramine). If you have a history of seizures, you should be careful with this combination. Ask for clarification from your prescriber.
None. Other than both medications cause sleepiness they are unrelated and can be taken together. Do not drive or operate machinery.