"At the end of each day you will fill out a short survey with a few questions evaluating the effects of the pills."
Self-reporting is an unsuitable mechanism to draw out scientific results.
There's an excellent detailed explanation available (1) but in TL;DR here are four of the most compelling factors at work:
1 - Honesty/Image management
2 - Introspective ability
3 - Understanding / Question Interpretation
4 - Response bias
Take Image Management & Response Bias - participants know that they will be able to see their results vs. the control group and it's not a leap to realize how easily our ego and even subconscious need for validation could dramatically skew the full study results.
(1) http://www.sciencebrainwaves.com/the-dangers-of-self-report/