Acute intermittent porphyria (AIP) has been associated with schizophrenia in
some studies, but prior research is limited by the absence of comparison
populations. Here, we linked Swedish registers to examine the risk of
schizophrenia and bipolar disorder in 717 individuals diagnosed with AIP and
their first-degree relatives, compared with matched individuals without AIP
and their first-degree relatives. Individuals with AIP had a fourfold
increased risk of schizophrenia or bipolar disorder. Similarly, relatives of
individuals with AIP had double the risk of schizophrenia or bipolar
disorder, suggesting that these associations may be as a result of common
genetic influences.