No "fast" method for gaining weight can be called safe. However, if you wish to gain a good amount of weight within a reasonable time, add home-made butter (available for purchase in many places), cheese, eggs, milk, and meat to your food. However, I must caution you, do not overdo it. Try to keep it to 1 to 2 kilograms of weight per month. Do not try to attain more than 10% of your optimmum weight.

I would think that just like losing weight, you'd want to check with your doctor, because everyone's metrics are not the same.  For example, if you are diabetic, you will have certain dietary restrictions, and the same applies if there is a concern about cholesterol.

There are products like weight gain shakes you can drink. Just make sure they aren't nothing but sugar drinks. The best type of weight you can gain is muscle. Remember muscle weighs more than fat does, so the more muscle you gain, the more weight you gain. Gaining fat isn't healthy. If you're young and you need to gain weight for sports, remember that what you gain today could turn into unhealthy fat in just a few years. Here is an article about weight gain.

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Weight Gain Tips: How to Put on Weight Safely

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After all my research on weight gain, it seems a good old-fashioned peanut butter and jelly sandwich on whole wheat bread sum it up!

Try incorporating more healthy carbs such as oatmeal in your diet. Bananas are known to increase body weight so try including them in the diet through a smoothie along with your regular protein powder. Weight training also helps bulk up and add to weight.

Think about your goals for gaining weight. If you are too thin because you don't have enough muscle, do weight training. Look for resources to help you learn to lift safely and effectively. The be sure to get enough protein to support muscle development, and to increase your overall increased caloric needs; muscle burns more calories than does body fat. 

If you are really thin,the process of gaining weight depends upon your constitution,your height,your food habits etc.However,you have not revealed, whether you are a vegetarian or a non-vegetarian, what is your present weight and how much weight you want to gain actually etc etc.It is quite tough to answer your vague question with so many ifs and buts.It is better to consult a doctor and a dietician as well.

Eating More Often

For gaining weight, you’ll start consuming huge amount of calories. But your body has the capacity for assimilating only limited calories during each of your meals. The absolute effect of calories getting absorbed by your body happens when you reduce the meal size and subsequently increase the meal frequency. 

Exercise is not only for those people who are trying to lose weight, It is also recommended for those people who are trying to gain weight because without proper exercise the weight that they’ve gained will all turn to fat which is not healthy to know muscle gain truth. If your chose to excesses with weight lifting, it is important to start gradually so that you will not overwork your body, three to four times a week and 30 minutes to one hour per session should be enough. Consulting an instructor in a local gym will be one good idea so that you can following the proper routine and achieve a desire result.

Check out this link  Top 10 ways to gain weight where you will find simple and effective ways to put on weight quickly and healthily. Also read Weight Gain Diet: Best Foods to Put on Weight.