Salt Peter is sodium nitrate, NaNO3 , a natural, white crystalline compound used in solid rocket propellants and in the manufacture of explosives, tobacco and incense.
In the past, Salt Peter was put in military rations to keep soldiers from being interested in sex. Priests are also thought to be fed meals spiked with Salt Peter to insure celibacy. Desperate woman have been known to feed their husbands Salt Peter to keep them from straying, however, this practice is not recommended. Feeding a man Salt Peter may keep him from cheating, but it will also stop him from having sex with his mate. A better idea would be for a woman to take steps to make herself so irresistible, working on emotional levels, that he has no desire to be with another.
For fidelity, rather than feeding your loved one Salt Peter, try placing a sachet beneath his bed, but also take steps to bind your love and encourage sex.