Gels are semisolid systems consisting of small particles suspended in a liquid that is given structure through the addition of a gelling agent. This causes a high degree of physical or chemical cross-linking that results in high viscosity and a semi-solid state. Gels can be single or multiphase depending on if the components are uniformly distributed or separated. Common gelling agents include carbomers, cellulose derivatives, and natural gums. Gels are used topically in products like acne treatments, anti-inflammatories, and vaginal medications due to their ability to maintain drug levels at application sites.