Marijuana use may increase the risk of ectopic (tubal) pregnancies, researchers at Vanderbilt University Medical Center (Dey et al.) reported this week. (Internet Medical Journal Monday 27th September)

The researchers studied CB1, a "cannabinoid" receptor that binds the main active chemical for marijuana, delta-9-tetrahydrocannabinol (THC).

In pregnant mice that lacked the gene for the receptor, or in which the receptor was blocked, the embryo failed to go through the oviduct – the tube leading from the ovaries to the uterus. The same thing happened in normal mice when the receptor was over-stimulated.

The study, published in the current issue of the journal Nature Medicine, describes for the first time how the CB1 receptor in the mouse regulates muscle contraction to move the embryo down the oviduct.

It is not known whether drugs that block or, in the case of marijuana, over-stimulate the CB1 receptor can cause ectopic pregnancy in humans. However, "our results raise caution for women of reproductive ages regarding the chronic use of marijuana for recreation or pain alleviation," the researchers concluded.