Psoriderm cream contains two active ingredients, distilled coal tar & lecithin. Coal tar has been used in skin conditions for hundreds of years, but it is still not fully understood how it works, mainly because it is made up of so many different compounds. Coal tar has mild antiseptic & anti-inflammatory effects, & relieves itching. It is also absorbed into the skin cells & is thought to inhibit DNA replication in these cells. This slows down cell division & stops the cells from multiplying excessively. In turn, this reduces the thickening & scaling of the skin & clears up psoriasis plaques. Coal tar may also work by breaking down keratin, a protein that forms part of the skin structure. This helps the skin cells shed from the treated area, thus reducing the thickening & scaling of the psoriasis plaques. Lecithin is a phospholipid that is included in this medicine to soften psoriasis scales & enhance the absorption of the coal tar into the skin cells. Psoriderm cream is used to treat subacute & chronic (long-term) psoriasis, including psoriasis affecting the scalp & flexures, eg at the elbows, knees or wrists. The cream should be applied to the affected areas of skin or scalp once or twice daily, or as directed by your doctor.