Since the drug quantity given to mice depends on the animal weight:
1. How often we have to weigh the animal to adjust? Once a week?
2. Is it acceptable if in the same group that has around 8 mice to have the average of the weight of the 8 mice and give the same drug quantity based on the average weight of the 8 animals?
Dear Abdelaziz
For a mouse, I am sure you can get away with weighing only once a week. However, I would not recommend you to have an average of the weight as one mouse in the group might be much larger due to dominance. I would suggest weighing all the mice.
Also depending on what you are doing to the mice. If you are doing a dietary study, I would recommend weighing daily. Maybe monitor the weights of the rats for the first couple of days and depending on the weight gain make a decision. I am sure weekly weighing of each individual mouse should be fine.
I agree with Maritza. I think weighing mice twice weekly is ideal and the weight gain/ loss could be drastic sometimes with the issue of dominance.
I would suggest weighing the mice individually rather then taking the average.
It will depend on whether you are using males or females...also housing conditions and enrichment can have a substantial effect on body weight. Assuming you will eventually weigh organs, brain weight will also be different depending on these conditions.
I would like to add an important issue related to that of weighing each animal individually. For this type of experiments you should individualize all data, i.e. you should be always able to know which data comes from which animal. In my experience, this important aspect is not always taken into account by the researcher performing the experiments. And at the end of the experiment it is too late and one has -quite probably- to repeat everything again.
In my experience, drugs are delivered in a vehicle. In this way you would have a specific amount of drug in a specific volume of liquid so that you can adjust the volume per weight of animal. Yes, animals will be receiving different amounts of raw drug but it will be the same per body weight, i.e., 20mg/kg of body weight.