Objective: A study on chromosomal abnormalities has raised concerns that medication with methylphenidate (MPH) for attention-deficit/hyperactivity disorders (ADHD) might increase the risk of cancer. We performed a rigorous test of the association between cancer and MPH and other drugs used for ADHD, based on data from nationwide Danish registers.
Methods: Data were linked from five registers containing information on a total of 21,186 patients with ADHD, their drug prescription rates, and associated cancer diagnoses between 1994 and 2010. The cohort included subgroups treated with MPH only, amphetamines only, other ADHD-specific drugs only, antidepressants only, antipsychotics only, mixed medication, and a control group of patients with ADHD who had never taken medication. Frequencies of cancer diagnoses in these groups were compared. In addition, hazard risk (HR) ratios for developing cancer, and survival rates in these subgroups, were analyzed.
Results: The mean observation time varied between 1.3 and 10.8 years for the various drugs. Cancer rates in the total group amounted to 1.27 per 10 000 person-years before and to 4.33 per 10 000 person-years after onset of treatment. The frequency of cancer was significantly higher (p = 0.05) after than before medication only in the antipsychotics subgroup. Furthermore, for mixed medication, the cancer frequency in a small subgroup was significantly higher (p < 0.05) after onset of medication than in the unmedicated subgroup. The Cox regression analysis indicated that none of the drugs represented risk factors, whereas higher dose (p < 0.001) and older age (p < 0.05) were risk factors for developing cancer.
Conclusions: The concern that children taking MPH and other drugs over long periods of time could be at a significant risk of developing cancer is not substantiated by these findings in a large and representative sample, which had been diagnosed and treated over a period of 17 years.