Alfalfa syrup is good in general to relieve from weakness&to increase ur Appetite.That itself only will not help in putting up wt.So,u need to focus on food that u eat...should be more proteinaceous...like Wheat,jawar,all millets,Milk daily minimum twice,Eggs white,Yellow aldo can taken,but not msny..as its fat.(if u r non.vegetarian),meat,fish related items also but not from outside like KFC etc,home made with less oil,Soya,Ragi,Bananas,all fruits,green leafy,vegetables.Try to  take all these as much as possible,provided regular exercises will make u fit,but just by eating u will put up wt but not fitness. Regarding Alfalfa...U can continue for few months.No problem.No side effects.