Supplements? Well for example some really popular ones:
Ma Huang / Ephedra (appetite suprresnt and energizers) - may give you a heart attack
Yohimbe (energizer, libido booster) - again may make your heart explode
L-Arginine (amino acid, nitro producer, precursor to HGH) - may cause an outbreak of shingles - especially if you are imuno deficient for any reason (HIV, Stress)
St Johns Wort (anit depressant, stress reliever, MAOI)- (here is the biggy) taken with an SSRI (Zoloft or even herbal Passion Flower) could lead to major issues including death
Tribulus, Damiana, Goat Weed (raises testosterone, frees testosterone) - prolonged use and prolonged high T levels show a strong correlation to prostate cancer.
Contraindications are the big thing - maybe an MD could weigh in. Dang - you can buy Cigarettes and they are not safe.
Dave Pendel - calls this the "Poison Path", the active study of psycho active (mostly plan life) substances. His primary pilosophy is don't do it - and if you have to be an expert first; Carlos Castenada - write of the powers of plants and the fear he had of them. Even subtle powers (recouperating quickly) - can have negative effects on your body if taken long term...
You only have one body and this life is a gift.