Injectable steroids are injected into muscle tissue, not into the veins. They are slowly released from the muscles into the rest of the body, and may be detectable for months after last use. Injectable steroids can be oil-based or water-based. Injectable anabolic steroids which are oil-based have longer half-life than water-based steroids. Both steroid types have much longer half-lives than oral anabolic steroids. And this is proving to be a drawback for injectables as they have high probability of being detected in drug screening since their clearance times tend to be longer than orals. Athletes resolve this problem by using injectable testosterone early in the cycle then switch to orals when approaching the end of the cycle and drug testing is imminent.
Though you may already be aware of potential side effects from testosterone abuse (note I said abuse and not use), here they are again: lowered HDL-cholesterol levels (good cholesterol), testicular atrophy, reductions in sperm count, prostate enlargement, liver damage (primarily with oral steroids that have been modified with a 17-alkyl substitution), menstrual irregularities, suppression of endogenous hormone levels (like LH and T), development of palpable breast tissue in men (also known as gynecomastia), clitoral enlargement, and acne.