Im a 23 year old male and antihistamines completely knock me out to the point where its hard to wakeup. im on the smaller side so could that effect it?

Different med: Most antihistamines are based on age rather than weight. Also, first generation (old) antihistamines commonly cause drowsiness. Most people do not have drowsiness with the new generation antihistamines such as claritin, (loratadine) zyrtec, allegra. However, for whatever reason, some people are sensitive to usual doses. A lower dose could be tried, or an allergist can assist with other therapies.