Much has been written about the ability of both medications and individual genetics to affect how people metabolize drugs. But did you know things like herbal supplements, smoking cigarettes and even grapefruit juice could have the same effect?

The U.S. Food and Drug Administration has warned, for example, that certain dietary supplements can boost the effects of other medications as well as change how the human body processes medications. This could change drug blood levels, meaning a patient could be getting too much, or too little, of the medication they need.

For example, birth control pills and drugs designed to treat HIV/AIDS, heart disease and depression are less effective when taken with St. John’s wort, an herbal supplement. Reduced or increased efficacy of certain medications could result in potentially harmful adverse drug events or treatment failures.

FDA officials also warn against combining supplements that thin the blood, such as ginkgo biloba and vitamin E, with warfarin (Coumadin), a common medication prescribed for cardiovascular disease or blood clots. Taking any of these products together may increase the risk for internal bleeding or stroke.

Research has also linked cigarette smoking with increased activity of the CYP1A2 enzyme, which processes many common medications, including the antipsychotics olanzapine (Zyprexa) and clozapine. Stopping or starting smoking could result in increases or decreases of CYP1A2 substrates, necessitating a change in dose.

Also in the CYP enzyme family are CYP3A4 and CYP3A5. These are the most common drug-metabolizing enzymes in the body and process roughly half of the most commonly prescribed drugs on the market, including opioid pain medications, statins, chemotherapeutic drugs and combined oral contraceptives.

In a brochure on pharmacogenetics, the American Medical Association reports common grapefruit juice to be a CYP3A4 inhibitor, meaning it decreases the enzyme’s ability to process drugs. This means that in people regularly drinking grapefruit juice, drugs that require the activity of CYP3A4 will not be metabolized at the same rate as in those who do not drink grapefruit juice regularly.

What’s more, people who are CYP3A4 intermediate metabolizers could see an even greater reduction in metabolism from regular consumption of grapefruit juice since their CYP3A4 activity is already compromised.

The YouScript Personalized Prescribing software database contains hundreds of substances, both prescription and not, and can detect potential interactions between them and many medications.