Have you ever wondered why your friend starts behaving like she’s drunk after only one drink while you can drink much more alcohol and only feel slightly buzzed? The term “lightweight” has come to describe someone like your friend – a person who does not need much alcohol to feel drunk. But how a person responds to alcohol – or any other substance for that matter – is determined by a number of factors. When prescribing medications, physicians need to consider many of the aspects below to determine the appropriate dose (Read Overdose: Why It Happens & What to Do When It Does). These aspects, alone or in combination, may also explain why two people have dramatically different responses to the same amount of alcohol, a prescription drug, or an illicit substance.

1) Age: Elderly and children may differ from adults in their response to substances. Children weigh less, and not all body systems may be fully developed. In contrast, the body systems of seniors may have slowed, and elderly may not be able to adjust to new medications as well as would their younger counterparts.

2) Weight: A person who weighs 200 pounds will likely require more of a substance than someone who weighs 150 pounds in order to achieve the same effect.

3) Race: Some research suggests that different racial groups have variations in metabolic patterns for certain substances. One study, in particular, found a difference between individuals of Asian descent and Caucasians; the former group demonstrated an increased sensitivity to alcohol.

4) Tolerance: Tolerance is a key concept in addiction research. Tolerance occurs when a person’s body begins to “tolerate” similar amounts of a substance and requires more of it (or a greater frequency) to achieve the same effect. Only certain substances are known to cause tolerance.

5) Time: All people are subject to changing biorhythms over the course of a day. Some of these biorhythms include sleep cycles, metabolism, and body temperature. In this way, taking a drug in the middle of the night when your body would expect to be asleep could create a different response than taking it during normal, waking hours.

6) Health Status: Sickness can change a person’s response to substances. This is especially true if the illness has affected the organs that are involved in metabolizing alcohol and other substances.

7) Hormones: Some studies suggest that men have increased amount of the enzyme that assists the body in metabolizing alcohol. This may be why women develop alcoholic liver disease more quickly than men.

8) Diet: Certain foods can affect the ways in which substances are metabolized. Fasting can change the rate that the body absorbs foods, and malnutrition can create enzyme deficits. Scientists who argue against racial differences in response to substances believe that these differences are really attributable to varying diet-related behaviors.

9) Administration: Method of administration is the way the substance enters the body and may include snorting (inhaling), smoking, oral consumption (swallowing a pill), and intravenous injection. Many people who abuse prescription drugs try to circumvent their timed-release properties by grinding them into a powder and snorting or injecting them.

10) Psychological Variables: Numerous psychological variables can play a role in how a substance influences a person. Many clinical trials test the “placebo effect,” which occurs when a person is treated with a harmless substance but believes that he/she is receiving medication and, as a result of the belief, improves. Furthermore, social influences can affect the way a person responds to substances.