8.5 Drug interactions

A Family Guide to Concurrent Disorders - Part III: Treatment

A drug interaction occurs when one drug alters the action or effects of another drug also present in the body. Some interactions are trivial, while others can be dangerous and possibly life-threatening.

Drugs taken together:

• can act independently of each other. For example, alcohol does not seem to interfere with the action of vitamins or oral contraceptives, or vice versa.
• can increase each other’s effects. This could happen because they affect the brain in the same way or because one drug increases the concentration of the other in the body. For instance, alcohol and antihistamines are both central nervous system depressants. Therefore, the combination can increase both the desired effects of the drugs (e.g., disinhibition or decreasing of self-control) as well as the side-effects (e.g., drowsiness).
• can decrease each other’s intended effects (an antagonistic effect). This could occur when one drug “blocks” or prevents another drug from producing its effect. It could also happen when two drugs have opposite effects on the brain (e.g., alcohol-induced drowsiness versus caffeine-induced alertness).

Consequences of drug interactions

An important factor in choosing a medication for someone with concurrent disorders is to pay attention to potential toxic interactions between the prescribed medication and the drugs of abuse. This is important in case the person relapses on the substance while taking prescribed psychiatric medication.

In some cases, drugs do interact but the result does not noticeably affect the person. How-ever, combining substances of abuse, which can have toxic effects on their own, with prescription medications often has consequences that are much more serious. The most common toxic effects are:

• central nervous system depression, which, at its mildest, is drowsiness, but in its more severe form, can lead to a coma
• respiratory depression, which can lead to the person stopping breathing altogether
• cardiac effects, such as blood vessels constricting or dilating, or changes in heart rhythm that can lead to the heart stopping
• decreased seizure threshold, meaning that the brain can have a seizure more easily
• psychiatric effects, such as psychosis.

Some of the more common combinations and their effects are listed below.

Stimulants and antidepressants

If a stimulant, such as cocaine or methylphenidate, is taken with a monoamine oxidase inhibitor (MAOI) antidepressant, a hypertensive reaction (high blood pressure) can occur. There have been many reports of rapid onset of headaches and severe hypertension when amphetamines are consumed by people who are taking MAOIs, some with fatal outcomes from cerebral hemorrhage (bleeding in the brain). The combination of a stimulant and a tricyclic antidepressant can cause an increased heart rate.

Cannabis and antipsychotics

Marijuana can decrease the effectiveness of antipsychotic drugs and increase the risk of relapse. Taken with certain antipsychotic drugs, marijuana can also lead to marked hypo-tension (low blood pressure) and increased disorientation. There can also be additive effects with anticholinergics and other medications that have anticholinergic side-effects, leading to such symptoms as increased dry mouth, urinary retention and constipation.

Tobacco and antipsychotics

Smoking decreases the blood concentration level of certain antipsychotics by 20 to 100 per cent, so smokers require higher doses. If a person reduces or quits smoking while taking an antipsychotic, the blood concentration levels of the drug will increase, leading to increased side-effects and possibly toxicity. Therefore, dose decreases are usually required, but must be monitored closely.

Tobacco and benzodiazepines

Some of the substances in tobacco can stimulate the liver enzymes that metabolize diazepam and chlordiazepoxide. This causes them to clear from the body more quickly, leading to a need for higher doses.

Caffeine and lithium

Caffeine acts as a diuretic, which can worsen incontinence. This affects water balance and can therefore affect lithium levels. Caffeine can also increase excretion of lithium from the kidneys and lead to lithium tremor. People who take lithium may still have some caffeine in their diet, but it is important that they do not drastically change their intake from day to day.

Next >>>

• Print Bookmark