I think the difficulty with these debates is the phrase 'drug use' is an emotive/ loaded phrase in its own right, as we've seen in the various debates on here, despite the fact that it covers an incredibly wide range from someone who has a spliff once a month to someone who's mainlining crack on a daily basis.
In particular, those who are against any form of drug use tend to take a very hard line about it. Partly I think this is because it's illegal and people (rightly or wrongly) believe that if something is illegal then it is bad by definition and there can thefore be no middle ground - any amount of illegal drug use is wrong. Partly it is because many people know someone who has had severe problems as a result of drugs and therefore similarly but subjectively views any amount of illegal drug use as carrying the same risk. Partly it is because there is very relatively little readily available objective public information available - most data or evidence which comes out is immediately politicised by pro or anti-drug campaigners.
E.g. The cocaine industry causes deforestation - at 300,000 hectares per year it does according to Colombian Vice President, but it also ranks a long, long way behind logging, mining, oil and gas, cattle farming, plantations, dams and other engineering projects, fuelwood, fires and even tourism according to the World Rainforest Movement.
E.g. Just one pill can kill you - it can and it can do so rather quickly, but with 16 deaths from Ecstacy in 2006 out of an estimated 26 million tablets taken in the same year (Source: ONS, Guardian), the actual risk of is a miniscule 1 in 1.6 million vs a 1 year risk of 1 in 400,000 of dying in plane crash. Even the odds on being struck by lightning are not that much higher at 1 in 2.0 million.
E.g. Modern strains of drug (e.g. skunk) are far more potent than before and are likely to have a more damaging impact on mental health - they are more potent in terms of TCH content and in theory, regular usage of higher dosage drugs should have a more negative impact. On the other hand, the number of people admitted to consultant care with drug related mental health or behavioural problems has fallen over the last 10 years by 4% (according to the NHS).
I'd also add that people who are pro-drugs can also be very black and white, their arguments typically coming down to either (a) I've used various sorts for ages and have never known anyone including myself to have any ill effects and (b) alcohol is just a 'legal' drug, almost everyone uses it and it causes many more deaths than illegal drugs which actually may just be an argument to regulate alcohol a lot more.
I think what is clear is that from a personal health standpoint, the absolute risk from drugs is relatively low. There can be virtually no question about that. With over one third of the UK adult population having taken drugs at least once in their lives and 10.5% having taken them in the last year alone, the incidence of death or physical or mental illness resulting is truly minimal as evidenced by many sources, including government reports. Equally, whilst use of recreational drugs clearly does have a negative impact on global society, this will clearly vary depending on the type of drug, e.g. Ecstacy is typically manufactured, distributed and consumed within its own country. Equally, being purely objective, from say an environmental and sociological perspective, I suspect the global drugs trade has a lesser impact than many other industries, e.g. petrochemicals, mining, logging, biofuels and pharmaceuticals. If you're boycotting drugs solely on these grounds, there are probably many, many better products to be boycotting first.
Personally, it what is acceptable probably comes down to the individual. If you are anti-drugs then any personal health risk, however small, or wider impact, however small, is too much. If you are pro-drugs, or at least ambivalent, then objectively there should be a level of drug use which you could consider as being acceptable.