Many studies have shown that garlic is highly beneficial. IN the above study were anyone of the subjects tested to see if they were allergic to gralic or sulphur or any of the other compounds that are found in garlic.? I bet not. Its easy to show that something doesn't work if you bias the research. The person doesn't state how many subjects weren't affected by it. And yes it could have an effect if you overdose on it just like any other food. You need to look at the whole study, looking at signficance, biases and how they came to their conclusions before you can make such a bold statement.
Garlic has personally helped me come out of a dark hole when I have been unable to move. It does quite the opposite to me - it makes me more alert and able to see and think, relieves pain and nausea to name a few benefits. Just remember even the best studies are flawed. Make up your own choice. If we studied all food looking for the negative side you can could claim that each has its negative effects. Have you ever heard of everything in moderation!