Intrusive thoughts are not inherently bad, especially if you can identify them as random and unhelpful to your daily life. But they can become problematic when they accompany other mental health issues or cause you to act out in unhelpful ways.
Obsessive-compulsive disorder (OCD): People with OCD experience intrusive thoughts that create significant distress. These intrusive thoughts might include fears of contamination, a need for order, and aggressive thoughts about losing control. You then engage in compulsive, ritualistic behaviours to try to mitigate these disturbing thoughts.
Post-traumatic stress disorder (PTSD): Intrusive thoughts in PTSD may look like reliving the traumatic event (having flashbacks or nightmares) or experiencing ongoing anxiety about the trauma happening again.
Avoidance behaviours: Some people with intrusive thoughts assume that a specific disturbing thought indicates reality. For example, if you fear that you might hurt your baby sister, you avoid spending time with her altogether. Or, if you fear that you'll swerve your car into ongoing traffic, you might avoid driving to prevent this from coming true.
Eating disorders: Intrusive thoughts about weight gain, poor body image, or eating can cause and maintain eating disorders. Engaging in disordered eating can also trigger more intrusive thoughts about control and perfectionism.
Anxiety disorders: People with anxiety disorders often have intrusive thoughts related to losing control or something bad happening. Anxiety can cause you to miscalculate a minor stressor to be a significant catastrophe.