Because dietary supplements are not as strictly regulated as drugs, their effectiveness may not be as dependable. Furthermore, when taken in dosages that exceed the recommended dietary intake, some supplements may be toxic and cause serious side effects. Vitamin A toxicity, for example, can cause liver damage, blurred vision, headaches, bone pain and swelling, drowsiness and nausea. Supplements can also interact with medications, which can cause unwanted side effects and decrease the efficacy of the medicine.