While most of its salts are insoluble, thallium(I) sulfate is soluble and gives a colourless, tasteless solution that is an insidious poison, much used by the Iraqi security forces to dispose of those who have fallen from favour. In some countries it is still used to kill vermin, sometimes with disastrous results.

The most notorious thallium poisoner was the serial killer, Graham Young, who in 1971 put thallium sulfate into his workmates' coffee at Bovingdon in Hertfordshire. Many were taken ill and some died of the mysterious 'bug'. The case was famous because it was the first time that a poison had been detected in cremated remains, which were analysed by atomic absorption spectrometry and proved to have much higher levels of thallium than expected.

Because thallium(I) ions are like potassium ions they are assimilated into the body and affect the potassium-activated enzymes of the brain, muscles and skin, producing symptoms of thallium poisoning. The antidote is the ink dye, Prussian blue, which exchanges its potassium for thallium and carry it from the body.