Assumption of Homoscedasticity not met...Repeated measures ANOVA

Hey guys,

Student in clinical research here and in dire need of help. My study is a clinical trial with 2 treatment groups measured over 4 time points...with the outcomes assessed being blood levels.

So my independent variables are "treatment" (2 groups) and "time" (4 time points - with time being a within-subjects factor)...and my dependent variable is "blood levels"

So...I'm doing a repeated measures ANOVA with 2 levels (mixed model) in SPSS
BUT I'm finding that a major assumption isn't met.

My data meets the assumption of normality and sphericity...but not of homogeneity of variances/homoscedasticity. I read somewhere that minor deviations of homoscedasticity are excusable as long as the largest variance is less than 4x the smallest -- my data does meet this requirement but is this true?

I'm wondering what my best options are...given this limitation.

I've tried transformations and they haven't really helped this.

I'd really appreciate any suggestions/help. Thanks!