Here is the list of top supplements people buy when trying to gain mass:
- Protein
- Creatine
- Glutamine
- Multi-Vitamin
- Natural Test Booster
- Prohormones
- HMB
- Growth Hormone
- NO2
- Anti-Estrogens
- Protein Bars
- Amino Acids
- Methoxy, Ecdy
- ZMA
My suggestion is sticking with the top 4 on that list. Whey, micronized creatine, glutamine, and a multivitamin. Since you are obviously familiar with the protein and creatine, I can post some info on what it does for the body. If you have any questions about the other supplements listed, feel free to ask

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Glutamine
L-Glutamine is the most abundant amino acid found in muscle tissue. It helps prevent muscle wasting and improves recovery. It also helps protein synthesis. The better and quicker you recovery, the sooner and harder you can hit it in the gym! Glutamine is safe to talk year-round.
Glutamine should not be taken at the same time as creatine because they compete for receptors to be absorbed. Take glutamine pre-workout and creatine post-workout.