ELI5: Why can't people take steroids long-term for autoimmune diseases?

People cannot take steroids long-term because, while they quickly stop the immune system from attacking the body, they eventually cause too much damage to other healthy organs and bones.

Think of your immune system as a neighborhood watchdog. In an autoimmune disease, the dog gets confused and starts biting the family instead of protecting the house. Taking a steroid is like giving the watchdog a very strong sleeping potion. It immediately stops the dog from biting you, which makes you feel so much better right away.

However, you cannot keep the watchdog asleep forever, and the sleeping potion itself creates a big mess in the house if used every day. In everyday life, this medicine affects almost every part of your body:

• Weakened bones: It makes your bones brittle like dry chalk, so a simple slip on the sidewalk could break a hip.
• Sleeping guards: With the watchdog completely knocked out, real burglars—like germs, bacteria, and viruses—can walk right into your home without any fight, making you get sick very easily.
• Sugar spikes: It changes how your body handles food, which can cause high blood sugar or lead to diabetes.
• Mood swings: It can make your brain feel like it is on a wild rollercoaster, causing you to feel angry, super anxious, or unable to sleep at night.
• Puffy changes: It makes the body hold onto extra water and fat, often changing your appearance by creating a very round, swollen face.

Instead, doctors use steroids for a short time to put out the "fire," and then switch to gentler, long-term medicines that can gently train the watchdog to behave without knocking it out completely.