Medical visualization of a nitric oxide molecule. Nitric oxide is an important signaling molecule, and is so small, that unlike other neurotransmitters, it can diffuse quite far and act on several nearby neurons, not just those within the synapse connection. Nitric oxide causes relaxation of smooth muscle and increases blood flow, thereby playing an important role in digestion as well as the development and maintenance of erection. Nitric oxide doesn't affect blood flow directly, but instead initiates a series of chemical reactions that allow the blood vessels to dilate. The most important milestones along this chemical pathway are two proteins known as cyclic GMP (cGMP) and PDE 5. cGMP is the protein that signals the muscles in the blood vessel walls to relax and fill with blood. PDE 5 breaks down cGMP to prevent overstimulation of the blood vessels and excessive relaxation. If not enough cGMP is made or if cGMP is broken down too quickly, the blood vessels won't remain relaxed long enough to fill with sufficient blood to allow the penis to become erect. Current erectile dysfunction drugs work by blocking the effect of PDE 5 and allowing cGMP to remain active longer. These drugs can have beneficial sexual effects for men-and, some studies have found, for women. They make it easier for men to both achieve and maintain an erection, and for women to experience the vaginal lubrication and the enhanced clitoral sensitivity associated with normal sexual arousal.

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