You won't find it down an aisle of the supermarket or in a bottle in your medicine cabinet. Your doctor can't prescribe it and you can’t buy it for your birthday. But without it you could be more susceptible to the common cold and more vulnerable to depression, heart disease and drug and alcohol abuse.

It's self-esteem, a reflection of how much you value, appreciate and approve of yourself. A healthy self-esteem means you like yourself, believe you deserve love and happiness, and feel confident in what you can accomplish.

But if you're plagued by low-self esteem, chances are you have an inner critic living rent-free in your head, one that whispers (or shouts), "I'll always be alone," "I'm stupid and boring," "I'm useless."

Regular verbal beatings such as these, along with a lack of confidence, a reluctance to trust your instincts and opinions, and treating yourself badly could be signs that it's time to improve your self-image. Think a little more Donald Trump and a little less Woody Allen.

"What's (self) love got to do with it?"
Besides making you feel worthless and unlovable, low self-esteem is hazardous to your health. The negative emotions or moods it triggers, such as anxiety and depression, can increase the risk for heart disease. How? They wear down the emotion-sensitive immune system and are associated with increases in inflammation, which has been linked to heart disease.

Low self-esteem can raise blood pressure and lead to unhealthy behaviors such as smoking, excessive drinking and avoiding social contact.

Low self-esteem can sap your motivation to take care of yourself. If you don't like yourself very much, blowing off steam with a six-pack after a bad day looks a whole lot more appealing than jogging six miles. Studies have shown that people with high self-esteem are more likely to exercise regularly.

"Don't worry…be happy"
A healthy self-esteem is an important key to positive emotional states. That's what experts call joy, contentment, feeling relaxed and gratitude. These positive states help buffer you against stress and they contribute to emotional and physical well-being.

Shakespeare was onto something when he wrote "Mirth and merriment…bars a thousand harms and lengthens life." Modern science is beginning to confirm the Bard's wisdom. Here are some recent findings linking positive emotions to good health:

• Laughing and coping by using humor improved immune function and increased the level of an immune system protein, the body's first line of defense against colds.
• Positive emotions reduced the readmission rate of people hospitalized with heart disease.
• Optimism (having a positive outlook and being able to bounce back from bad events) cuts the risk for heart attacks by half and has been linked to better recovery from heart bypass surgery.
• Positive emotions help counteract the body’s reaction to stress.
• Positive emotions produce more flexible, creative and efficient thinking.
• Positive emotions are associated with better sleep.