Hope your concentration is doing better!
Although herbal remedies may help with short term situations... Make sure your other priorites are in order first.
Are you getting enough sleep? How often do you exercise? Exercise increases your circulation thus improving the thought process. Are you under a lot of stress? Is there pain that you're dealing with? Getting a regular/ weekly or monthly massage will help with circulation and stress relief allowing you to be more alert in the lecture.
Finally, ask yourself if you are interested in what the lectures are about... Is what you are reading what you want to be reading? If not, nothing is going to help you concentrate. Find things you like to do and are interested in. If these are lectures in college (??) you may want to find a different major or switch to a different class. Sometimes the professors make it difficult to follow along.