drug calculation



Can someone please help me with the following drug calc. I am a bit stuck!!

Mrs. A was prescribed 800mg. of metronidazole tds. Pharmacy has supplied suspension with a dosage of 500mg/ml.

How much would you administer to this resident?

a) per dose- 800/500=1.6 (is this correct???)

b) in 24 hours- how do I calculate this one??? could someone please please help me out..

Thanks a million :)


2,453 Posts

Not sure what your question is.

You have figured out the volume needed to administer 800 mg.

Then you give that volume TDS.

What exactly is the question?

nursej22, MSN, RN

3,285 Posts

Specializes in Public Health, TB. Has 38 years experience.

I am not familiar with the abbreviation tds. Can you clarify?


6,593 Posts

a) Correct and don't forget to always include units of measurement

b) You need to know what tds stands for. Do you know? The problem is super simple if/when you do know.


11 Posts

TDS= three times a day


3,413 Posts

Has 38 years experience.

TID was three times a day when I worked the floor.


11 Posts

I am from new zealand thats why its different

Tenebrae, BSN, RN

1 Article; 1,849 Posts

Specializes in Mental Health, Gerontology, Palliative. Has 11 years experience.

I am from new zealand thats why its different

Hey fellow kiwi

Good to know all the abbreviations, I've worked in some places that use TDS and some use TID, if you arent sure, take a moment and look it up

The one nursing calc that helped me hugely was

what you want Amount it comes in

----------------- X -----------

What you have 1


Good for some practice and it has the answers at the end


11 Posts

Hey good to see :bugeyes: another Kiwi out here :yes:

yep but at Counties Manukau we mostly used TDS.. but I have heard about TID too!!

KelRN215, BSN, RN

1 Article; 7,349 Posts

Specializes in Pedi. Has 16 years experience.

Real world answer = call the pharmacy and ask why they didn't send tablets for an adult. Metronidazole comes in 200 mg and 400 mg tablets.

Specializes in MICU. Has <1 years experience.

As I learned it, the equation is desired over have multiplied by the quantity.

So we have how much you want (800mg) divided by how much you have in the bottle (500mg) multiplied by the quantity, being 1mL.

It helps to keep that multiplication step in there even if it's not necessary here, just to stay consistent. So you've got it, 1.6mL :D

Since TDS means three times a day, you'll take your answer and multiply it by 3

Specializes in ED, Pedi Vasc access, Paramedic serving 6 towns.
Real world answer = call the pharmacy and ask why they didn't send tablets for an adult. Metronidazole comes in 200 mg and 400 mg tablets.

Mrs A. broke her jaw and it will be wired the next day. Seriously had the pharmacist ask me this at a CVS when I brought prescriptions in for liquid Clindamycin (sp), prescribed in liquid by my oral surgeon for that reason. Does the pharmacist really think the doctor and myself are that stupid that we couldn't have figured out that normally pills would be easier. I just rolled my eyes and let them know my jaw was broken and was going to be wired shut the next day... I think they realized what a stupid question it was!!

So maybe Mrs. A is in the same predicament! LOL