Four out of five smokers who quit gain weight, according to “Smoke Free,” a publication funded by the American Cancer Society. Although the health benefits of quitting smoking far outnumber the risk of weight gain, the fear of adding inches to your waistline may dim your motivation to give up your smoking habit.

Adderall is a stimulant drug that can only be obtained with a prescription. Although the medication is legitimately used for the treatment of attention deficit hyperactivity disorder, it is also abused for non-therapeutic purposes.

All the biochemical processes your body does daily make up your metabolism. These processes consist of basic bodily functions -- such as pumping blood -- physical activity and digestion. The energy used to fuel these processes is measured as calories, which you get from food.

Many men are aware of the large number of products available to enhance the libido. However, these drugs and supplements can put you at risk for numerous side effects. For instance, Viagra can cause flushing, headache and stomach upset.

Anorexia nervosa, often called simply "anorexia," is a serious psychological disorder characterized by extreme weight loss through self-starvation, and an unshakable belief that you are still fat no matter how much weight is lost.

A lot of factors beyond your control help determine your basal metabolic rate, which is the number of calories your body needs to perform basic functions like breathing. Your age, gender and genes, for example, play a role in setting your BMR.

Transit time is the rate at which food passes all the way through your digestive tract – from ingestion at the mouth to elimination.

Your metabolic age will give you an idea of what kind of physical health you are in using your base metabolic rate, or BMR. If your metabolic age is lower than your actual age, it means your body is in good health.

Klonopin, the brand name for clonazepam, belongs to a class of drugs called benzodiazepines. Klonopin is mainly prescribed to treat anxiety, panic attacks and seizure disorders. Klonopin works by enhancing the activities of neurotransmitters that reduce excess brain activity.

Venlafaxine, sold under the brand name Effexor, is an anti-depressant medication. Effexor is a serotonin-norepinephrine reuptake inhibitor, or SNRI. It's a class of drugs that target the neurotransmitters serotonin and norepinephrine in your brain.

Tetrahydrocannabinol, or THC, is the active ingredient found in marijuana or cannabis. Marijuana smokers have long wondered if THC increases metabolism.

Underactive thyroid, or hypothyroidism, is a condition that significantly slows your metabolism. Your thyroid gland, located at the front of your neck below your voice box, is part of the endocrine system.