What does "incidence not known" mean for side effects?

When manufacturers list side effects under “incidence not known,” it means that there is not enough data to determine how often they occur. Side effects listed in this section of a product label means that some people have reported them after taking that particular medication. Based on these reports, it cannot be determined if these side effects are truly due to the medication or another reason.

Many times, side effects are listed with unknown incidence from postmarketing reports. These are voluntary submissions of side effects that people experience to the manufacturer after the medication came on the market. Because they are not reported from controlled clinical trials, there is no way to determine a true incidence for these side effects in the general population.

Even though the incidence of some side effects may not be known, it does not mean that the side effect is rare or harmless. If you experience any side effects, tell your healthcare provider. They can provide guidance on whether the side effect is related to the medication and recommend appropriate steps. They can also help you report any possible side effects to the appropriate surveillance systems.