A number of prescription and over-the-counter medications can cause constipation. If you're currently taking any medication, you might want to ask your doctor or pharmacist whether it could be causing your constipation. Among the drugs that can cause constipation are calcium-channel blockers taken for high blood pressure, beta blockers, some antidepressants, narcotics and other pain medications, antihistamines (to a lesser degree), certain decongestants and some antacids. Antacids that contain calcium or aluminum are binding and can cause constipation; antacids that contain magnesium tend not to cause constipation. If you are unsure what's in your antacid, check the label or ask your pharmacist or doctor.