A drug with a __________ therapeutic index has a higher likelihood of causing adverse reactions.

A drug with a low therapeutic index indicates that there is a narrow margin between the effective dose and the lethal or toxic dose. This means that only a small increase in dose can lead to adverse reactions, making it more likely for such drugs to cause negative effects when dosed improperly. Drugs with low therapeutic indices require careful monitoring and precise dosing to avoid toxicity.

In contrast, drugs with a high therapeutic index have a wider safety margin, meaning they are less likely to cause adverse reactions because the effective dose is significantly lower than the toxic dose. Moderate or variable therapeutic indices may have varying impacts on safety but do not specifically imply a higher likelihood of adverse reactions like a low therapeutic index does.