I think you may be asking about what are sometimes called "smart drugs" or Nootropics
I can give you information on these medications, but be aware that using medication to enhance performance is controversial, and there are not many doctors or physicians that will prescribe these medications for that purpose, only. There are several reasons for this. First of all, there is often an underlying medical or psychiatric reasons that one does not feel able to 'keep on top of things'. Such conditions as hypothyroid, chemical depression, vitamin and mineral deficiencies, can cause these symptoms. Lack of adequate exercise and good sleep can be a factor in poor mood and concentration as well (and is very common today).
Additionally, one must be very careful, because while these medications may initially provide a person with what seems like a boost, this is often a short term response - often over time, this effect wanes. This can lead to addiction problems, as the individual will take more of the drug, trying to get the same effect as when they first took the medication, or combine one drug with another in a dangerous manner.
Many of the drugs that are considered 'nootropics' are amphetamines or of a similar class. These drugs can have dangerous side effects such as increasing blood pressure and/or causing psychosis. So even if one is found to have a condition for which these medications could be prescribed, they will need to be monitored by a their physician. For example, a person that has psychosis, will not be aware that this problem is occurring. A person with chronic elevated blood pressure can develop permanent damage to the heart, kidneys and circulation (including the brain's circulation).
Also, these drugs are most often not approved by the FDA for 'performance enhancement' . This means that a doctor might lose their license if they were found to prescribe medication for indications that had not been approved by the FDA.
This Wikipedia article on Nootropics lists medications and supplements for performance enhancement. Double Click HERE
My suggestion for you would be to first
see a physician for physical exam and explain the symptoms you are having that problems with focus and concentration, and see what they offer you.
Additionally, if you are eating mainly a diet of pre-prepared foods or processed (boxed, or wrapped foods that only need to be heated or unwrapped) it would be suggested that you start working on improving your diet through eating fresh meals that you prepare your self. There are many micro nutrients that science isn't even yet aware of that are found in fresh fruits, vegetables, and other whole foods. There are courses you might take at a health food store, for example if you don't know where to start with improving your 'fueling' (diet).
Importantly, if you are not exercising 5 to 7 days per week for an hour, ask your doctor for an exercise plan so that you can work up to this amount (you will not want to start at an hour of vigorous exercise, but work up to it). You might be very surprised at how much vigorously exercising can improve one's concentration and focus.
Another thing to be aware of is that most people need at least 8 hours of sleep per day (it is easiest to achieve this if you maintain a similar bed time and wake time most days - and vigorous exercise will aid you in getting a good night's rest as well) . The brain requires sleep, and without it one will have difficulty concentrating and focusing. In fact it has been found that poor sleep (either interrupted or shortened amounts) is directly correlated to inability to focus and concentrate, as well as can lead to conditions such as depression and obesity.
If you have any problem accessing the wikipedia link with the list of medications, or have any further questions or need clarification of anything, please reply so that I can assist you further!