The 1840 edition replaced the plural effects with the singular effect, in agreement with the following singular verb form is. This reading was followed in the RLDS textual tradition until the 1908 edition. The original plural effects was restored in that edition since the printer’s manuscript reads in the plural. Joseph Smith did not make that change in his editing of 𝓟 for the 1837 edition, so the plural effects was left unmarked in 𝓟. The singular effect first appeared in the LDS text in the 1907 vest-pocket edition; all subsequent LDS editions have followed the singular reading. Another possibility would have been to change is to are and to leave effects in the plural.
In the current text, there is only the singular effect. There is one other passage where an original plural effects has been edited to the singular:
In this second case, the original manuscript is extant and reads in the plural, despite the occurrence of the singular subject and verb, it is. Note, by the way, that in Alma 30:16 the is could not have been changed to the plural are without also changing the subject it to they. Also note that the RLDS text did not restore the original plural effects in Alma 30:16, yet 𝓟 itself reads in the plural and was not corrected in 𝓟 to effect. In any event, this second example argues that the use of “effects is” in Mosiah 7:30 is intended; the critical text will restore effects as the original reading.
Summary: Restore in Mosiah 7:30 the original use of the plural effects in “the effects thereof is poison”; similarly, Alma 30:16 originally read “it is the effects of a frenzied mind”.