The rare-earths, including thulium, have very low toxicity. However some lingering effects due to the repeated intravenous injection have been reported, including liver and spleen degeneration, as well as fluctuations in the hemoglobin concentration. The damages in the liver due to the thulium ingestion (0,01 to 1% of the diet) are more prominent in males rather than females, according to experiences performed with mice.