Thenes is totally correct regarding his various posts.... First of all, buying whey protein and L-Glutamine supplements are perfectly healthy ways to help you reach your fitness goals, including putting on weight and more specifically muscle. These two products, while readily available at many health supplement stores, gyms, etc are also sold directly in pharmacies. If you are worried about the content of some of these supplements, I would advise you to buy directly from a pharmacy where there are nutritionists and pharmacists on hand to answer any of your questions. The difference between these two locations is twofold: first, the incredibly ripped guy at the counter of the health supplement store may be able to tell you which products help you gain weight fast, but he probably hasnt been trained in chemistry, biology or other sciences. His knowledge about side effects, complications with other medicines, etc would be very limited as compared to a trained pharmacist. Second, pharmacies are held to a higher standard. The products they sell are usually more limited, but its the ones that are proven to be safe. Most are approved by Health Canada (the equivalent of the FDA) and probably have similar certifications in the States. You wont accidentally be buying anything that is "harmful" to you if you take the pharmacy route.
Also, Thenes was bang on about muscle atrophy. Like anything else, if you stop practicing, you'll get rusty. With your muscles, this is no different. If you train hard for 6 months or a year or even 5 years and then you take a substantial amount of time off, you will lose muscle mass. You need to talk to a nutritionist or personal trainer about what your goals are, how to achieve them, and then what to do once you achieve them. Not every guy out there is looking for the bodybuilder image where you are always looking to get bigger, and thus you need an exit strategy after getting the body you want. If your goal is to put on a few pounds of muscle and lower your body fat percentage, then once you reach this plateau, you need to reduce the amount of calories you take in (from fat, protein and carbs) to about the same amount of calories you exert in a day. (While gaining weight you obviously want to have a higher intake than output). For example, once you get to your goal of gaining 10lbs of muscle and reducing your body fat percentage by 5 points, you need to continue doing physical activity, including weight lifting (although maybe not as intense as before) and have a solid diet that will adequately meet the daily requirements of your new body.
Lastly, the whole point of supplements is that they SUPPLEMENT your diet and work out needs. Depending on your diet, goals, timelines, etc, trainers will suggest you get between 1g and 1.5g of protein per pound of body weight. Ie. if you are 150lbs, you need between 150g and 225g of protein per day while working out. Considering most single servings of meat (beef, chicken, turkey, etc) contain less than 30g of protein, its very hard to consume this much protein on a daily basis (and costly as well). I know of people who take the "natural" approach and have up to 6 breasts of chicken per day, tons of nuts, beans, lentils, tuna, etc. and the results are ALWAYS two fold: they put on a lot of muscle, but they put on a lot of body fat as well. One great thing about whey protein supplements is that they are usually about 90% protein. These other high protein foods are generally under 30% protein, the rest being either fat or carbs. In order to get 225g of protein the natural way, a person ends up consume 6000-7000 calories per day. This is what Michael Phleps requires for his swimming training in which he spends 8 hours in a pool! The average male burns about 3000 calories a day and with a good work out plan and cardio, most guys dont burn over 4500 calories a day. Whey protein is a great way to keep the protein-calorie ratio where you want it without getting a lot of extra fats and carbs.
I was a varsity track and field sprinter for my university here in Canada for 5 years and I competed all over North America and Europe. I've had numerous coaches, trainers, nutritionists, sports doctors, kinesiologists, etc over the years and they have all said pretty much the same thing: plan out your diet, consume 5-6 smaller meals a day instead of 3 large ones. Figure out how many calories you burn in a day based on height, weight, physical activities, etc. (internet sites are useful for this) then figure out how many calories you need to consume to reach your goals. Then count EVERYTHING and be very strict about your diet. Your diet is about 60% of what it takes to reach your goals, with weight lifting, cardio, good nights sleep, etc making up the other 40%. Good healthy food is important, and make sure you are getting at least 50% of the protein you require from natural foods. This is why its called "supplements".