To determine whether toothpaste manufacturers and mouthwash manufacturers are competitors, we can look at the cross-price elasticity of demand for the two products. If the cross-price elasticity is positive, the two products are competitors.

If the cross-price elasticity of two products is positive, it means that when the price of one increases, the demand for the other increases. We would expect to see this with competing products as customers look to buy a competing product when the price of what they were previously purchasing goes up.