Physical and psychiatric health conditions affect the lives of many autistic adults. However, relatively little is known about individual trajectories in autistic individuals' physical and mental health from adolescence to adulthood.This study uses a well-characterized longitudinal sample (Seizure onset continued well into adulthood, with two individuals experiencing their first seizure at the age of 25 years. Seizures and neuropsychiatric medication use were both higher for those with lower intelligence quotient (IQs). In addition, medication use increased over time for these individuals, whereas those with higher IQs saw a reduction in medication use with age. Between the ages of 15 to 30 years, the predicted probability of medication use increased from 82% to 88% for those with lower IQs and decreased from 37% to 13% for those with higher IQs. Collectively, almost 70% of all participants in this study were classified as either overweight or obese. Body mass index (BMI) increased throughout adulthood, especially for those with higher IQs. The steepest increase in BMI over time occurred for those with higher IQs who also took antipsychotic medications.Overall, continued risk for developing seizures, high rates of neuropsychiatric medication use, and significant and increasing rates of obesity from adolescence to adulthood underscore the importance of monitoring health issues in autistic individuals and those diagnosed with other neurodevelopmental conditions throughout the lifespan.Autistic individuals and those diagnosed with other neurodevelopmental conditions are at risk for many physical health issues from adolescence to adulthood. This includes, but is not limited to, seizure disorders, high rates of medication use, and obesity. Yet, not much longitudinal research exists measuring these health issues throughout the lifespan and, especially, in adulthood.The researchers analyzed data from a sample of 253 individuals who lived in North Carolina, Michigan, and Illinois. This sample was recruited almost 30 years ago, beginning the study when they were roughly 2 years old. Some participants had an autism spectrum disorder diagnosis, and some did not, but instead had other neurodevelopmental conditions. The researchers measured changes in medication use, height, and weight beginning in early adolescence (age 10 years) into adulthood (age 30 years). The researchers also tracked seizure occurrences across the life span (beginning at age 2 years up until age 30 years).In adulthood, this sample showed high rates of obesity and medication use. For example, almost 70% of this sample was classified as either overweight or obese. In addition, 55% of this sample was taking at least one neuropsychiatric medication in adulthood, and 22% of this sample was taking three different types of medication at the same time. Individuals with higher and lower intelligence quotient (IQs) had different findings over time: obesity increased throughout adulthood for those with higher IQs and medication use increased throughout adulthood for those with lower IQs. Seizures first developed for a group of individuals in adolescence and adulthood. Two individuals experienced their first seizure as late as 25 years of age.Research, and especially longitudinal research, on physical health in autistic adults is limited. Although we know that autistic individuals generally experience high rates of obesity, seizure disorders, and medication use, less is known about how these physical health issues change over the lifespan. This study has provided useful information regarding how certain physical health issues change over time, and who these physical health issues impact the most in adulthood.The findings in this study identified that, in adulthood, autistic adults experience high rates of physical health issues, including increasing rates of obesity and medication use for certain populations. These findings confirm how important it is for autistic adults to have access to quality health care across the lifespan. We hope that, with these findings in mind, caregivers and health professionals will work to ensure that autistic individuals, from childhood through adolescence and adulthood, will have access and assistance in monitoring these ongoing and persistent health problems.