Why People Respond Differently to Substances

You and your friend go to a party. After 2 drinks, she's slurring her words, but you're fine, and you know from experience that you could drink a lot more without a problem. Why does this happen? Everyone's body and mind represent a unique combination of physical and psychological factors that result in different responses to substances like alcohol, prescription medications, and even illicit drugs.

In the case of prescription medications, your doctor needs to consider all of these factors to determine an appropriate dose for you. In the case of illegal drugs, these factors may account for why two individuals with the same level of tolerance may have dramatically different responses to the same dose: for one, a high – for the other, a lethal overdose

What factors determine how a substance affects an individual?

Weight: A person who weighs 250 pounds requires more of a substance than a person who weighs 125 pounds to achieve the same effect.

Race: Research indicates that certain racial groups have metabolic differences, which can determine that group's response to a given substance. For example, studies suggest that individuals of Asian descent have an increased sensitivity to alcohol when compared to their Caucasian counterparts.

Age: Children and the elderly may need medication adjustments for a number of reasons. Young children are physically smaller (and, by extension, weigh less) and all relevant body systems may not be fully developed yet. On the other hand, some of the bodily systems of seniors have slowed, and they may have more difficulty adjusting to or recovering from certain amounts of substances and/or side effects.

Tolerance: Tolerance occurs when an individual requires larger and/or more frequent doses of the same drug to achieve or maintain the original effect. Not all substances cause tolerance.

Hormones: Research indicates that women have less of the enzyme that helps metabolize alcohol in the stomach than men do. Because women have higher concentrations of alcohol in their blood than men do after consuming a similar amount, women may develop alcoholic liver disease more easily than men. Other studies in animals have suggested other differences in responses based on sex or hormones, but further research is necessary to determine to what extent this information is applicable to humans.

Illness: Diseases that damage bodily organs, especially those involved in metabolizing substances like the liver and kidneys, can affect how a person responds to a certain amount of a substance.

Diet: Some foods alter metabolism of certain substances. Additionally, fasting and malnutrition can modify the rate of absorption and create enzyme deficiencies, respectively. Some scientists believe that what others understand to be inherent racial differences is really the result of culturally determined nutritional habits.

When: Time can play a role in how a substance affects a user. Everyone is subject to naturally occurring biorhythms. For example, metabolism can vary depending on time of day. Other biorhythms include sleep cycles and body temperature.

Method of Administration: The effects of a substance also depend on how quickly (and in what amount) it enters the bloodstream. Many prescription medicines deliver small doses regularly over a period of time in order to provide continual relief for pain. Someone who abuses prescription medicines may grind the tablet into a powder and snort it (inhale) to circumvent the time-released properties. Intravenous (IV) injection (via a syringe) also delivers substances more quickly than if they had to go through the normal digestive process.

Psychological Variables: A number of psychological factors affect an individual’s response to a drug including the expectations of the self or others. For example, research indicates that, in clinical trials for which a patient is given a completely harmless substance in a pill without being told that the substance is harmless, the patient may have negative side effects or positive results, all a product of their mind. This is called the placebo effect. Additionally, consuming substances in the presence of friends compared to doing so alone or in another environment can have different effects as the user’s body adjusts to the expectations (or lack thereof) of a given social situation.

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