Re: What are the affects of drugs on plants?

I doubt if plant scientists have examined the effects of all human pain 
relievers on plant growth with the exception of aspirin. Aspirin 
(acetylsalicyclic acid) is converted to salicylic acid which appears to play a 
role in plant response to stress. Leslie and Romani (Plant Physiology 
88:833-837) reported that salicylic acid inhibits ethylene biosynthesis. 
Ethylene is the stress hormone. For a review of salicylic acid and plants see 
Ann. Rev. Plant Physiol. Plant Mol. Biol. 43:439-463. 

Some key factors in any experiment are replication (number of plants per 
treatment), how growth is determined, and the environment for each plant. One 
plant per treatment is usually not enough. If the plant dies, you are not sure 
if it was the treatment or some other factor. However, with five plants per 
treatment, if one of five plants died in one treatment and the other four were 
healthy, then you would be relatively sure it was not the treatment. Height 
alone is often not the best way to measure plant growth (Hershey, 1995). Were 
the plants all exposed to the same amount of light, the same temperature, 
received the same amount of water and fertilizer, etc. so the only factor that 
differed was the drug? 

Another thing to consider is other ingredients in the pills you used. They may 
have had a positive or negative effect on plant growth.

 
Reference

Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley.