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Painkillers half as effective when you take testosterone

You're in the middle of a course of testosterone, and you develop a raging headache. Never mind, you think. That's what paracetamol is for. It's just a shame that the makers of paracetamol didn't take men who are injecting testosterone into account. According to an animal study done 25 years ago, paracetamol has less effect if you're injecting testosterone.

Painkillers half as effective when you take testosterone
In 1985 pharmacologists at Gandhi Medical College in Bhopal published in the Indian Journal of Pharmacology the results of a study in which they had done pain tests on rats. The test they did involved immersing the animals' tail in hot water. Then they measured how long it was before the rats pulled their tail out of the water. The longer it took, the less sensitive the animals were to pain.

This is how the researchers tested the pain-relieving effect of paracetamol and indomethacin. They administered the painkillers a few minutes before doing the hot water pain test. The researchers' main aim was to find out more about the mutual relationship between the painkillers and the amount of male sex hormone circulating in the animals' body. So they also gave the rats testosterone injections, a dose of 1 mg/kg/day, beginning three days before the start of the pain test. The researchers also did their tests with castrated rats.

The results of the tests are shown below. You can see the average amount of time it took for the animals to retract their tail from the water. The times are in seconds.

Painkillers half as effective when you take testosterone

The testosterone level in group 4 was probably similar to that of group 1.

Paracetamol increased the amount of time that the rats kept their tail in hot water by 72 percent. If you give the animals testosterone, the amount of time they kept their tail in the hot water is only 32 percent longer. In other words, the testosterone injections reduce the painkilling effect of paracetamol by 55 percent.

Fortunately taking testosterone reduces the likelihood that you'll need a painkiller. A few years ago biologists at Princeton University discovered from trials on sparrows that testosterone implants reduce sensitivity to pain.

Indian J. Pharmac. 1985 17:136-9.