Side effects of activated charcoal are generally mild and infrequent. However, it can interfere with the absorption of certain medications if you take them at the same time. Activated charcoal may also cause delayed bowel movements. And when you do go to the bathroom, you may notice that your stool is black. This is normal and nothing to worry about. If you’re new to activated charcoal, consult a healthcare professional before use. This is especially important if you take other medications.