Does l-cystine work?

Overall Effectiveness Rating for L-Cystine

The overall effectiveness rating for l-cystine is 2 out of 3. This rating means there is some evidence that using l-cystine may lead to positive results. This rating also means that some evidence does not back up all claims. Below you can find individual ratings for specific claims.

Overall Confidence Rating for L-Cystine

There are 2 studies in the database on l-cystine; the confidence rating is 20. A score above 80 means the effectiveness rating for this supplement is reliable. A score under 80 means there is insufficient evidence to ensure a reliable effectiveness rating. Note, this rating is for l-cystine as a whole. There are also individual ratings for the claims below.

Individual Claims

The overall ratings above do no necessarily reflect each claim about l-cystine. Individual claims can have higher or lower ratings. For example, some supplements may have a lot of backing for one claim, but be completely useless for another. Click on claims below for more information.

Immune Health Claims Effectiveness Rating Confidence Rating
improve immune function
reduce frequency of the common cold


Title of Study
Effects of L-Cystine and L-Theanine Supplementation on the Common Cold: A Randomized, Double-Blind, and Placebo-Controlled Trial
Effects of oral supplementation with cystine and theanine on the immune function of athletes in endurance exercise: randomized, double-blind, placebo-controlled trial