Steroids: they’re infamous  for their use in sports. But they’re also found in inhalers,  creams to treat poison ivy and eczema, and shots to ease inflammation. The steroids in these medicines aren’t  the same as the ones used to build muscle. In fact, they’re all based on  yet another steroid— one our body produces naturally,  and we can’t live without. Taking a step back, the reason  there are so many different steroids is because the term refers to substances  with a shared molecular structure, rather than shared effects on the body. Steroids can be naturally occurring  or synthetic, but what all steroids have in common  is a molecular structure that consists of a base of four rings  made of 17 carbon atoms arranged in three hexagons  and one pentagon. A molecule must contain  this exact arrangement to be a steroid, though most also have side chains— additional atoms that can dramatically  impact the molecule’s function. Steroids get their name  from the fatty molecule cholesterol. In fact, our bodies make steroids  out of cholesterol. That fatty cholesterol base  means that steroids are able to cross fatty cell membranes  and enter cells. Within the cell, they can directly  influence gene expression and protein synthesis. This is different from many other types  of signaling molecules, which can’t cross the cell membrane and have to create their effects  from outside the cell, through more complicated pathways. So steroids can create their effects  faster than those other molecules. Back to the steroids  in anti-inflammatory medications: all of these are based on a naturally  occurring steroid called cortisol. Cortisol is the body’s  primary stress signal, and it has a huge range of functions. When we experience a stressor— anything from a fight with a friend,  to spotting a bear, to an infection or low blood sugar— the brain reacts by sending a signal from  the hypothalamus to the pituitary gland. The pituitary gland then sends a signal  to the adrenal glands. The adrenal glands produce cortisol,  and release some constantly. But when they receive the signal  from the pituitary gland, they release a burst of cortisol, which spurs the body to generate  more glucose for energy, decrease functions not immediately  related to survival, like digestion, and can activate  a fight-flight-or-freeze response. This is helpful in the short term,  but can cause undesirable side effects like insomnia and lowered mood  if they last too long. Cortisol also interacts  with the immune system in complex ways— depending on the situation, it can increase or decrease  certain immune functions. In the process of fighting infection, the immune system  often creates inflammation. Cortisol suppresses the immune system’s  ability to produce inflammation, which, again,  can be useful in the short term. But too much cortisol  can have negative impacts, like reducing the immune system’s ability  to regenerate bone marrow and lymph nodes. To prevent levels  from staying high for too long, cortisol suppresses the signal  that causes the adrenal glands to release more cortisol. Medicinal corticosteroids channel  cortisol’s effects on the immune system to fight allergic reactions,  rashes, and asthma. All these things are forms  of inflammation. There are many synthetic steroids  that share the same basic mechanism: they enhance the body’s cortisol supply, which in turn shuts down  the hyperactive immune responses that cause inflammation. These corticosteroids sneak into cells  and can turn off the “fire alarm” by suppressing gene expression  of inflammatory signals. The steroids in inhalers and creams impact only the affected organ— the skin, or the lungs. Intravenous or oral versions, used  to treat chronic autoimmune conditions like lupus or inflammatory bowel disease,  impact the whole body. With these conditions, the body’s  immune system attacks its own cells, a process analogous to a constant  asthma attack or rash. A constant low dose of steroids can help keep this renegade immune response under control— but because of the negative psychological  and physiological effects of longterm exposure, higher doses are reserved  for emergencies and flare-ups. While an asthma attack, poison ivy welts,  and irritable bowel syndrome might seem totally unrelated,  they all have something in common: an immune response  that’s doing more harm than good. And while corticosteroids  won’t give you giant muscles, they can be the body’s best defense  against itself.