If you ever look closely at a medication label, you may notice that sometimes the drug causes side effects that are the exact opposite of its purpose. For example, an anti-nausea medication may lead to a side effect of nausea. This is called a paradoxical drug reaction: an outcome that is opposite from the drug’s intended outcome.
These reactions can either be adverse or beneficial. For example, estrogens usually trigger ovulation when following the body’s normal hormonal cycle. However, when estrogen is taken for longer periods of time, it actually suppresses ovulation, which is the basis for birth control pills.
Many different types of drugs can lead to paradoxical drug reactions. According to a meta-analysis (an analysis of several past research studies) of reports from France about paradoxical drug reactions, the most common types of drugs that led to these reactions were psychotropic drugs (antidepressants, antipsychotics, anti-anxiety medications, etc.) and immunomodulating agents (substances that enhance or reduce the immune response and are often used to treat some cancers).
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The exact cause of paradoxical drug reactions is still unknown, but there are a few potential reasons that researchers have hypothesized. One possible explanation is hormesis, which is when something that may be harmful at high doses is beneficial at low doses. For example, ionizing radiation is very dangerous in high amounts because it damages DNA leading to mutations. In lower doses, however, it can be used as a treatment for cancer. Therefore, researchers hypothesize that the reason we see paradoxical reactions is that the typical dosage for a medication might be beyond some people’s personal thresholds for a beneficial response. Still, hormesis only explains some types of paradoxical effects, and much of the evidence supporting this theory comes from animals or cell studies, not clinical studies on humans. There are many different interactions in a human body that simply cannot be recreated unless the experiment is done on humans.
Additionally, there are a lot of factors involved in the body processing drugs, including the state of the body before taking the drug, genetics, drug tolerance, and present infections, that can all play a part in the body’s reaction. There seems to be a general consensus among researchers that there isn’t just one mechanism that can describe paradoxical responses, because there are so many different interactions in the body that cause them. While paradoxical reactions are still not completely understood, they generally do not present a large risk to most people. It still remains a fascinating phenomenon that hopefully will be better understood in the future.
Featured Image Credit: Haley Lawrence
