Why doctors instructed to take some medicines after food and some before food?

Medicines absorbed into our body system by interfering with food. So the amount of drug that enters the blood and reach the site of action is altered. Based on this mode of action doctors prescribe them to take before or after.Some drugs may be known as not to cause any side reaction when taken in empty stomach and can be fully absorbed by blood stream. This kind of drugs to be taken before food. Some medicines should be taken along with warm plain water, to avoid any reaction between components in milk, curd or tea. But, most of the drugs are advised to take after food. Because it may irritate the stomach by increasing the acid secretion leads to discomfort or pain, chest burning, nausea and vomiting. To avoid these, drugs should be taken after food.