Psoriasis tends to thicken into patches and can last for years or decades. You'll need to manage your symptoms with your doctor.
Lichen planus may stop with simple red bumps that often go away on their own, though they sometimes return. And unlike psoriasis, lichen planus can affect the inside of your mouth.
You can't spread either condition to someone else.
Psoriasis seems to start with your genes. You're more likely to get it if one of your relatives has it. The condition causes your immune system -- your body's defense against germs -- to misfire and send faulty signals that make skin cells grow too fast.
Genes don't seem to play a role in most cases of lichen planus. And it's not even clear what happens in your body to cause the condition, though doctors suspect a mistaken immune system response may have something to do with it.
People with hepatitis C -- an infection that causes liver problems -- appear more likely to have lichen planus. Your doctor may test your blood for the virus that causes hepatitis C if you have symptoms of lichen planus.
Some medications can cause skin symptoms that look like lichen planus, especially diuretics for high blood pressure, heart disease drugs, and anti-malaria medicine. If you're seeing a dermatologist, a specialist who treats skin problems, let them know about all the medicines you take.
Psoriasis can cause symptoms that range from simple red spots to the dry, thickened, flaky patches of skin that are the most common sign of the condition (plaque psoriasis). It also tends to cause bits of skin to flake off, sometimes enough that you notice little pieces around your home and workplace.
Lichen planus is more likely to be limited to smaller lesions or spots and doesn't commonly cause flaking. But sometimes thicker, scaly, patches can show up, especially on your lower legs and ankles.
Though both conditions can appear anywhere on the skin, psoriasis is most common on the knees, elbows, scalp, and the crease of your behind.
Lichen planus is more common on your inner wrists and inner forearms. A type of the condition called "hyperkeratotic lichen planus" is thicker, looks more purple and wart-like, and is more common on the lower legs and ankles.
Psoriasis and lichen planus are itchy, but if you have lichen planus, the itchiness is typically more intense.
Your dermatologist will take a full health history and ask you about your symptoms, whether you notice anything that worsens or triggers them, and if you have family members with psoriasis.
They'll take a close look at your skin, nails, and scalp. If your doctor suspects lichen planus, they might look inside your mouth for white, lacy patches that don't hurt, especially on the inside of the cheeks. They also might test your blood for hepatitis C.
To confirm lichen planus, your doctor might remove a small piece of your skin to look at under a microscope, a procedure called a biopsy. This, along with blood tests, can help rule out other conditions.
You treat lichen planus with ointments and creams that you put on the skin. These usually have corticosteroids in them that ease swelling and itching. If that doesn't work, your doctor might give you a shot of corticosteroids.
Lichen planus is sometimes hard to treat, and your doctor might try a number of treatments to find one that works.
Psoriasis is also first treated with creams and ointments. These usually work quite well. Doctors sometimes recommend ultraviolet (UV) light therapy for psoriasis.
For stubborn psoriasis, your doctor might suggest medications that you take as shots or pills that work throughout your body to help fix the root cause of your symptoms: your body's immune response.