even tho ye, u may not be born with cancer cells, cancer is a mutation of DNA in many genes leading to unsupressed mitosis (division of a cell leavin behind the original cell and a new one which has the exact same characteristics of the last), but mitosis is nothing more than cellular growth and the reason behind physical development (growth). The problem arises when genes (proto-oncogenes, whose gene products promote cellular mitosis) become overactive and the cell division undergoes an apparently uncontrolled rate, but this alone will not be enough to cause cancer alone, as this is a normal part of the process of growth. Tumour suppressor genes create products which 'tell' mitosis to cease, and regulate cell growth, generally these genes are activated by cellular stress or DNA damage, and will further induce a process known as apoptosis (the deliberate cell death of unwanted cells in a multicellular organism, and the reason why we have individual fingers and toes), which decreases the risk of defective 'daughter' cells, but again, a mutation to only one tumor suppressor gene would not cause cancer either, due to the presence of many 'backup' genes that duplicate its functions. It is only when enough proto-oncogenes have mutated into oncogenes, and enough tumor suppressor genes deactivated or damaged, that the signals for cell growth overwhelm the signals to regulate it, that cell growth quickly spirals out of control. Cases in which cancer could not be 'caused' in the traditional sense (i.e smokin baccy or UV radiation from the sun) occur in children, proving that cancer, does not need to be caused, can, will, and does occur in otherwise healthy ppl, and will not always have to be sparked by the method described above (but will ultimately lead to mutation of DNA), as is seen in the case of estrogen dependant breast cancer (estrogen being a hormone, but also a carcinogen), which stimulates cells to divide and increase in number, increased cell growth leaves less time for DNA repair or the application of apoptosis, therefore increasing the risk of a genetic mutation (a normal part of cell division) to be passed to many daughter cells, if a sufficient number of defective cells accumulate then there is an increased risk carcinogenesis (malignant cancer growth), in other words ure fucked if you do, a lil bit less fucked if you don't, but it can still fuck ya.

As for ODing on THC, THC is pretty much the only chemical in the world to have a guesstimated LD50 (the dosage at which half the test subjects died) simply cuz its too high, but THC has a LD50 value of 1270 mg/kg (male rats), 730 mg/kg (female rats) orally (administered dissolved in sesame oil). If this were scaled up to an adult human, the lethal dose would be 2.8 ounces for a person weighing 140 lb (76 g for a 60 kg person, or over 1.4 kg [more than 3 lb] of average strength marijuana), rendering unlikely the possibility of an overdose, the therapeutic index (ratio of the quantity needed to experience psychoactive effects and quantity need for death) is 1:40,000 or 1:20,000 depending on where you look.
Fuk me i rambled there, soz I'm pissed