Epinephrine

Epinephrine is a hormone (a chemical substance released by cells to affect other cells in the body) that is naturally released by the adrenal glands in a stress situation. Its release results in increased heart beat, blood pressure and blood sugar levels, thus enabling the body to a "fight or flight" response.

Injection of epinephrine is a first aid measure upon an allergic reaction, because it will stimulate the heart beat and raise the blood pressure.

For more information see wikipedia.

Sheldon keeps Epinephrine around because he's allergic to bee stings. He offers it to Howard to ward off a possible peanut allergy incident (0102).