What Are the Differences Between Generic Cialis, Generic Levitra, and Generic Viagra? Buy Generic Cialis online, Buy Generic Levitra online, and Buy Generic Viagra online work by a similar mechanism to cause erections. There are subtle differences in how long the drug works and how quickly it works. Levitra works a little longer than Viagra buy online. They both take effect in about 30 minutes. With Levitra buy online, the effects last for about 5 hours. With Viagra, the effects last approximately 4 hours.Cialis works a bit faster (within about 15 minutes), and the effects last much longer -- up to 36 hours in some cases.If One of These Drugs Isn't Effective for Erectile Dysfunction medication, Can I Try Another?Yes, but because these drugs work the same way, it's unlikely that you'll have success with one if you've failed to achieve an adequate erection with another.What Precautions Should I Take Before Taking One of These Drugs?There are certain situations in which these drugs may not be safe to take. Before taking them, tell your doctor:If you are allergic to any drugs, including Viagra online without prescription or other ED medications.About any prescription or non-prescription medications you are currently taking, including herbal and dietary supplements.If you are scheduled for surgery, including dental surgery.If you take nitroglycerin or a long-acting nitrate to treat chest pain. The combination of Cialis, Levitra or Viagra with these drugs can cause dangerously low blood pressure.If you take alpha-blockers for blood pressure or prostate problems. The combination of Cialis online or Levitra online with these drugs can also cause dangerously low blood pressure.In addition, always follow the directions on your online pharmacy without prescription label carefully and ask your doctor or pharmacist to explain any part that you do not understand. Take these drugs exactly as directed. Do not take more or less or take it more often than prescribed by your doctor. Do not take these drugs more than once a day.Who Should Not Take Cialis, Levitra or Viagra? If you have suffered from a heart attack, stroke or life threatening arrhythmia (irregular heart rate) within the last 6 months you should discuss other options with your doctor. It is also advised to avoid these drugs if you have uncontrolled high or low blood pressure or if you experience chest pain with sex.