# How to calculate drug dosage



## dr.zapp (Dec 24, 2012)

I couldn't find this info in any stickies, and I answer this question frequently enough I think it needs to be in one, so here is how to do it yourself (and not need to rely on the online one ;-) http://www.ratclub.org/dose_general.htm. 

Start with the rat's weight in grams- I'll pick 400 g for example. Multiply that by the recommended dose (I get them from here- http://ratguide.com/meds/antimicrobial_agents/) in mg/kg (for amoxi that was 22 mg/kg, or 10 mg/lb). Because you need to keep the units the same, change 400 *g* to 0.4 *kg*, so the equation looks like 22 mg/kg amoxi * 0.4 kg rat = 8.8 mg amoxi rat (note how the kg units cancel out). So your 400 g rat needs 8.8 mg of amoxi each dose. How much of your 50 mg/ml solution is needed for 8.8 mg total? Take the amount you NEED (8.8 mg) and divide it by the concentration you HAVE (50 mg/ml) 8.8 mg / 50 mg/ml = 0.176 ml (the mg units cancel) so your 400 g rat needs 0.176 ml of a 50 mg/ml amoxi solution each dose.

To summarize- Recommended dose (mg/kg) * weight of rat (kg) / drug concentration (mg/ml) = amount given to rat each dose (ml) 

How to weigh a rat? There are many good kitchen gram scales on Amazon for ~ $10. Or you can go ghetto and use a hanger -http://lifehacker.com/5883540/approximate-the-weight-of-any-item-using-a-clothes-hanger


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## smopey (Feb 3, 2012)

Thanks for this! I always have trouble with dosages.


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## dr.zapp (Dec 24, 2012)

Here is a MUCH more thorough explanation http://www.ratballs.com/RatTails/Tails106.html


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## smopey (Feb 3, 2012)

dr.zapp said:


> I couldn't find this info in any stickies, and I answer this question frequently enough I think it needs to be in one, so here is how to do it yourself (and not need to rely on the online one ;-) http://www.ratclub.org/dose_general.htm.
> 
> Start with the rat's weight in grams- I'll pick 400 g for example. Multiply that by the recommended dose (I get them from here- http://ratguide.com/meds/antimicrobial_agents/) in mg/kg (for amoxi that was 22 mg/kg, or 10 mg/lb). Because you need to keep the units the same, change 400 *g* to 0.4 *kg*, so the equation looks like 22 mg/kg amoxi * 0.4 kg rat = 8.8 mg amoxi rat (note how the kg units cancel out). So your 400 g rat needs 8.8 mg of amoxi each dose. How much of your 50 mg/ml solution is needed for 8.8 mg total? Take the amount you NEED (8.8 mg) and divide it by the concentration you HAVE (50 mg/ml) 8.8 mg / 50 mg/ml = 0.176 ml (the mg units cancel) so your 400 g rat needs 0.176 ml of a 50 mg/ml amoxi solution each dose.
> 
> ...


Hi! Can you check my math? This is my first time trying to calculate dosages. I have a 250 gram rat (approximately) and I have 25mg amoxicillin pills. So would the equation be be.. 

22mg/kg amoxi * 0.25kg rat = 5.5mg

5.5mg / 25mg/ml = 0.22 Soo.. my 250mg rat needs 0.22ml of amoxi solution?

Thanks!


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## dr.zapp (Dec 24, 2012)

Yes that is correct IF you suspend the crushed up amoxi pill in 1 ml of whatever you are going to give it in (syrup, juice, baby food, etc).

Sent from Petguide.com Free App


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## smopey (Feb 3, 2012)

So you're saying I mix the amoxi with 1ml of flavoring and then her dose would be 0.22ml?


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## dr.zapp (Dec 24, 2012)

Yes, crush the pill up very fine, then put it in 1 ml liquid. Give 0.22 ml to your 250 g rat.


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