The dose ordered is diphenhydramine 50 mg IM every 4 hours
The available stock is 25 mg/2 ml
This means each ml of the stock solution contains 12.5 mg of diphenhydramine (25 mg/2ml = 12.5 mg/ml)
To calculate the volume to dispense:
Dose ordered divided by concentration per ml = volume to dispense
50 mg divided by 12.5 mg/ml = 4 ml
So the amount of diphenhydramine 25 mg/2ml stock solution to dispense for a 50 mg IM dose is 4 ml.
In summary:
Dose ordered: diphenhydramine 50 mg IM
Available stock concentration: 25 mg/2 ml (12.5 mg/ml)
Volume to dispense: 50 mg / 12.5 mg/ml = 4 ml
The amount to dispense is 4 ml of the diphenhydramine 25 mg/2ml stock solution to achieve a 50 mg IM dose.