Weakens adverbs are provided in this article. The words listed here are commonly found along with the verb weakens in sentences. This reference page helps answer the question what are some adverbs that describe or modify the verb WEAKENS.
actually
gradually
greatly
necessarily
seriously
Hope this word list had the weakens adverb you were looking for. In addition to adverbs used with weakens, other example verb adverb pairings can be found on this site.
You can help by adding more adverbs in the comments. Thanks.