Drug Math question

LIZPICURN · Jul 13, 2006

Hi, I won't just give answer, but I will walk it through with you. It is basically cross multiplying. If you have Demerol 75mg/2ml. You need to find out how many milligrams is in one ML. So go ahead and divide. (75/2 = 37.5. So there are 37.5 mg per ml of Demerol. Now, it is time to cross multiply. This is how I was taught and find it the easiest way to calculate meds. You take the mg/ml of the known concentration and cross multiply to solve for ?. You know you want to know how many ml's are in 25mg. You already know the concentration is 37.5mg per ml. Just by eyeballing you know your answer will be less than 1 ml.

_37.5 mg_ __25mg__

1 ml = Xml

Cross multiply to get 37.5X = 25.

Now divide by 37.5 to solve for X which will be your answer.

X= 0.66ml.

So 25mg= 0.66 ml

I hope this helps a little. If it doesn't, there are some great review books at your local book store or many sites on-line.

twinmommy+2, ADN, BSN, MSN · Jul 13, 2006

Ok I haven't had to do drug math in about 10 yrs, I took a test the other noc there were 53 questions 10 math question my score was 43, I did not get one math question right.
HELP
I can remember 2 question
demerol 75mg/2ml dr has ordered 25 mg ___ml to give?
heprin 100,000u in 500ml of fl to give 100units qhr and tubing gtt factor is 10?
can anyone tell me the formula to use for these?
I have the formula some where but I've put it up so good I can't find
You all ahve been very helpful in the past and so I thank you for your help now.

I forget about the second one, but the first one I'll take a stab at. Use ratio and proportion.

75mg in 2ml is equal to 25mg in Xml

written as

75mg:2ml::25mg:Xml

Pair up opposites across the equal sign (::) and multiply

25mg x 2ml = 75X

25 x 2 = 50 So... 50 = 75X

Get rid of the 75 in front of the X by dividing 75X by 75 and divide the other side by 75 as well to even the equation out.

50/75 = 75X/75

Leaves you with 0.66ml = X

OR2ER, BSN, RN · Jul 13, 2006

Desired Dose

___________ x no. of ml

Stock at hand

25

__ x 2 ml = 0.66ml

75

I still have to think about your second question.I don't have the formula in my hand.I'll be working tomorrow so i'll try to look for the manual and answer your question..