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The text on this page is taken from an informal compilation of opinions of contributors to the online VOLE List. As such, they are not peer reviewed and may contain differences of opinion. Those wishing to contact the list may contact Adrian Smith.

Has anybody got groups using tamoxifen as a cre lox inducer in mice? A group here is having toxicity issues when it is given 100mg daily for 4 days I/P in corn oil.

Yes lots of experience with tamoxifen. It's toxic stuff, especially at that dose rate. The other issue is that the oil that the tamoxifen is dissolved in causes problems for the mice when injected IP - they can develop peritonitis and fatty liver as a result. Just the amount of oil that's going to be in the mouse's abdomen after 4 x 100mcL IP doses will cause issues, so avoid this route.

Suggestions to refine the procedure:

* Ask the PI to check what dose of tamoxifen is actually required to switch the gene - this depends on construct and the tissue but often it's not the industrial dose that they think they need and that's often quoted in literature! (clinical doses of this drug are at around 0.25mg/kg!). Lowering the dose will help reduce the off-target effects too.

* Use mice that are mature (>17g) and in good body condition to begin with. Small transgenic and juvenile mice may not tolerate the tamoxifen so well.

* Give the tamoxifen/ oil mixture by the oral route, as the mouse's digestive system can cope with the oil better that way. If they're worried about administering an exact amount of drug, then Oral Gavage is OK, although more stressful than in diet.

*Otherwise, try tamoxifen in diet. It's not nice and goes rancid quickly: Keep in freezer and only take out the right amount of food for the mice for that day, then remove what's uneaten and top up fresh daily. Also flavouring the diet helps intake - banana milkshake seems to improve palatablity.

* Keep careful check on the mice and stop if they lose more than 15% weight. Dose / regime may have to be adjusted but in general, dose for the minimum amount of time to create the conditional state.

We've had a few groups reluctant to change from IP to oral route and who did pilot studies and are now converted, so it's worth persisting and persuading to make the refinement. ; - )

We have made an information sheet that we give to PIs that explains the above and a bit of background:


On the theme of toxicity, Huh et al. (2012) found stomach cell atrophy and metaplasia in mice following tamoxifen (note it occurred at much lower dose than used in gene induction).

It's a very odd drug to choose as the induction mechanism anyway, and alternatives should be sought.

Wyatt et al. (2022) highlight some of the things that have been mentioned: Tamoxifen administration induces histopathologic changes within the lungs of Cre-recombinase-negative mice.

Using a gene switch that is an independently active hormone, usually at many times the therapeutic dose, is asking for trouble. Add in the poor aqueous solubility necessitating oil mixture delivery that is often problematic and you have a system for gene manipulation that is a high welfare risk, and in my view should be phased out.

Indeed a very relevant discussion as the general concept that dosing tamoxifen is always safe remains a major challenge. But the sad reality is that many inducible GA strains rely on tamoxifen dosing- including in pregnant and neonates / juvenile animals , adding extra challenges. Some good developments on the oral dosing front and using purer formulations of Tamoxifen… but challenges remain: to get researchers to check for transgene expression with lower dosings!

We have seen swollen groins (only in males). We found the paper below where they report that the enlargement of the scrotum appears to be to be related to herniation and the presence in the scrotum of displaced fat and even intestinal loops. Se Reinert et al. (2012).

As well as swollen groins in males, mucometra occurs in females and urinary retention in both sexes.

It has been rightly pointed out that the tamoxifen inducible cre-loxP system is a powerful genetic tool. However the side effects are common and welfare impact high. Existing cre strains should be used with care and AWERBs should be asking tough questions when new constructs are proposed.