...mice were force fed catechins, a candy ingredient, compelled to swim in paint, then dissected.
Other experiments involved life-threatening cardiac puncture of mice with inadequate anesthesia.
Mars supported a cruel experiment to learn how a chocolate ingredient called PQQ affects metabolism by cramming baby mice into 200-milliliter Plexiglas metabolic chambers—around half the size of a 12-ounce soda can—and then submerging the chamber for nearly five hours in a chilled water bath, inducing labored breathing in the distressed mice. Experimenters then shoved tubes down the mice’s throats every day for 10 days to force-feed them the PQQ, after which they were killed and cut up for analysis.
Mars recently funded an experiment on rats at the University of California, San Francisco, to determine the effect of chocolate ingredients on the animals' blood vessels, even though the experimenter admitted that studies have already been done using humans. Experimenters force-fed the rats by shoving plastic tubes down their throats and then cut open the rats' legs to expose an artery, which was clamped shut to block blood flow. After the experiment, the animals were killed.
Usual "yeah, but"s probably apply. Pretty gruesome, though.