Tacalcitol is used to treat plaque psoriasis – this is the most common type of psoriasis. It is a condition where red scaly patches develop on your skin, called plaques. The most common areas affected are over your elbows and knees, the scalp and the lower part of your back. The patches may vary in size from time to time and can be itchy. Treatment for psoriasis aims to clear the plaques as much as possible.