Additives to the Drinking Water for Rats and Mice


It is often convenient as well as less interruptive to the animals to add medication as well as experimental compounds to the drinking water, rather than administering them by gavage or parenterally.  A critical factor to be determined when any compound is added to the drinking water is the animals’ fluid intake. It must be assured that fluid intake will not be diminished and the animals will not become dehydrated because of any new taste, smell, texture or other factor altering the drinking water.  This policy outlines procedures to be implemented for any and all drinking water additives.


A.   Any compound added to the drinking water must be approved in the IACUC protocol. 

B.   Ad lib water consumption must be determined before any compound is added to the drinking water.

C.  Compounds that are commonly used and with which the PI is familiar and can provide data or references that the water intake will not be reduced are exempted from pre-determination of ad lib water consumption.  These include, but are not limited to, sulfatrim and trimethoprim-sulfamethoxazole.  For compounds that have not been previously administered and for which data are not available, PI must provide data as described in II.A.  LASC/LACF will attempt to maintain a database allowing the information of tested additives to drinking water to be shared between investigators.

D.  If animals will receive a choice of unadulterated drinking water in addition to a water bottle with additive, as in a preference study, making a choice of drinking water possible, the study is exempted from pre-determination of ad lib water consumption.

E.  Monitoring for hydration by evaluating skin turgor, food consumption and body weight must be done for novel compounds and documented daily for minimum of 3 days.

 F.   The PI is responsible for labeling all drinking water with additives.

G.  A compound such as sucrose may be added to the drinking water in order to make the experimental compound or medication more palatable (II. D. 2&3).


A.  Evaluation of ad lib water consumption

1.   Prior to introducing a new compound in the drinking water PI must determine the strain’s typical 24 hour water consumption.

2.   The mean daily (24 hour) water consumption varies between species, stocks and strains.  It is therefore mandatory to determine the ad lib water intake for the same strain, sex, age and weight rodents as will be used in the study.  As an example, the mean daily water intake in mice has been reported averaging 7.7 +/- 0.3 ml/g body weight and varying 2-fold across strains.1

3.   Once the ad lib water consumption has been determined for one strain of mice or rats or other rodent species, ad lib water intake need not be repeated.

4.   Published data, or data from the planned LASC/LACF database, if available, may be used.

B.  Method for determining 24 hour ad lib water consumption.  Alternate methods may be considered as reviewed and approved by the IACUC.

1.   Rodents are housed in their usual cage. 

2.   A Special Care Instruction card stating “water is being measured DO NOT CHANGE WATER BOTTLE” is placed on the cage to prevent animal care staff from changing the water bottle or interfering with the study.

3.   The water bottle is filled (~ 200 ml-400 ml) and weighed using a digital scale to 0.1 g accuracy. 

4.   The same sipper tube as the animals are used to is used for the study. 

5.   The bottle with water is weighed 3 days in a row at the same time of day to determine 24 hour ad lib water consumption.

6.   The 3 measurements are averaged and then divided by the # of rodents in the cage.     

 C.  Adding the novel compound to the drinking water and documentation of fluid intake

1.   Volume of the water with additive consumed must be measured each day, as described above, and documented over a period of at least three (3) days.

2.   When a novel compound is added to the water, daily observations of the rodents to evaluate hydration and fluid and food consumption must be documented for the first three (3) days.

3.   If rodents drink less than 90% of their previous intake, become dehydrated or lose weight, PI must adjust the drinking water with additive so as to make it palatable for the animal.  If problem persists, PI must consult the veterinary services. 

 D.  Antibiotics added to the drinking water

            1.   Sulfamethoxazole/Trimethoprim (Sulfatrim)

Stock solution 200 mg Sulfamethoxazole +40 mg Trimethoprim /5 ml

MOUSE          Dose 95 mg/kg/24 hours  5.3 ml of stock in 400 ml drinking water.  

RAT                  Dose 53 mg/kg/24 hours  4.4 ml of stock in 400 ml drinking water. 2 

Suspension:      Shake bottle daily

Light sensitive:  Cover bottle with foil or dispense in colored bottle

2.   Doxycycline (as antibiotic)*

MOUSE          Dose 5-10 mg/kg/24 hours 5.3 ml of stock in 400 ml drinking water 

RAT                 Dose 5-10 mg/kg/24 hours 4.4 ml of stock in 400 ml drinking water  

Suspension:    Yes

Light sensitive:      Yes

Doxycycline is bitter and consequently it is advisable to add 2.0 – 5.0 % sucrose to the water.

*Doxycycline is also used as genetic modifier.  Dose to be determined by PI.

3.   Tetracycline

MOUSE          50-60 mg in liter drinking water 

Tetracycline is bitter and consequently it is advisable to add 2.0 – 5.0 % sucrose to the water.


1.   Bachmanov, AA, Reed, DR, Beauchamp, GK and Tordoff, MG. Food intake, water intake, and drinking spout preference of 28 mouse strains. Behav. Genet. 2002. 32:435-443. For detailed information please see

2.   Hawk, C. Terrence  Eds. Formulary for Laboratory Animals.  Third Ed. 2005.  Blackwell Publishing.  Rat and mouse doses calculated from allometric correction from large animal dose (30 mg/kg/day).

 BU IACUC approved May 2011