You should never assume you are sober and get behind the wheel just because another person who partook of the same amount of alcohol as you seems sober and can drive. For one, you shouldn't make a mistake just because someone else is also making it. Secondly, people get drunk at different rates; for example, you may get drunker faster than your buddy because:
You Drunk While Dehydrated
Dehydration makes you drunker than you are because it increases your blood alcohol content (BAC). Typical causes of dehydration include working out, being sick, or simply failing to drink adequate water. Therefore, if you have been exercising without drinking enough water and then went to the bar straight after that, your BAC is likely to be higher than a hydrated person who had the same level of alcohol. Therefore, your BAC may be marginally over the legal limit even if it could have been marginally below the level under ordinary circumstances.
You Weigh Less Than Your Buddy
Your weight affects your BAC; the more you weigh, the more alcohol it will take to raise your BAC. For example, if your buddy weighs 180 pounds, they can chuck in four drinks without their BAC going over the level limit. However, the same number of drinks would be enough to raise your BAC over the legal limit if you weigh 140 pounds or so.
You Are Sleep Deprived
Studies show that sleep deprivation has the same effects as alcohol intoxication. In fact, drivers are regularly advised not to drive while exhausted or sleepy. Therefore, if you start drinking while sleep deprived, you will appear drunker than you actually are. Sleep deprivation won't increase your BAC, but it will increase the intensity of your intoxication symptoms. For example, your reaction times will be slower, you may become more unsteady on your feet, and your speech may become even more slurred. As you can imagine, this makes you more likely to be arrested and charged with DUI.
In short, you should not compare your drunkenness with that of another person. In fact, the above factors are just three examples of why you may be drunker or appear more intoxicated with even a few drinks. Therefore, if you are arrested after sipping a little alcohol, don't assume you will go free. Consult a DUI lawyer from an office like Steven T. Fox Law Firm to help you with the case.