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Filter use, reuse and sharing

Though drug preparations may not be filtered at all by PWID (people who inject drugs), generally they are passed through a filter in order to eliminate impurities of the drug containing solution. In most countries, cigarette filters and/ or household items (cotton wool, cotton buds,…) are used. These items are not sterile and may not be clean.

Furthermore, filters are often reused and / or shared among PWID, not only because of a lack of filter availability, but merely because they get soaked with the solution to be injected and still contain some of the active drug. The retention of a considerable proportion of the active compound can incite PWID to hold on to their filter and to reuse it later or even to share or sell it (Bourgois & Shonberg, 2009; Scott, 2008; Keijzer et al., 2010). ).

Indeed, cotton reuse and sharing is very prevalent: in Scotland, 56 to 66% of the people kept their filter for later use and 34 to 42% gave their filter to someone else (Scott, 2008). In 5 different US cities, 54-56% of the people shared their cottons (Hagan et al. 2010) and Needle et al. (1998) found that 77% of the PWID reused their own cottons.

The Sterifilt® retains considerably less active drug than other filters. Scott (2008) stated that "The Sterifilt was found to retain 0.02 ml. The wheel filter retained 0.3 ml and the cigarette and hand rolling filters (…) both retained an average of 0.13 ml”. This low retention is reflected by the low quantity of active compound retrieved from the used filter, as shown in figure 1. For this reason, the Sterifilt has less potential for reuse, which seems to be confirmed by qualitative data (Scott, 2008 ; Keijzer et al., 2010).

Figure 1. Average amount of drug removed from used filters, shown in mg/ml of methanol which was used as a solvent for extraction (Scott, 2008)

The viral risks associated with filter reuse and sharing

Shah et al. (1996) have detected HIV antibodies, DNA and/or RNA on 18 to 36% of the used cotton filters they examined. The majority of the epidemiological research done does not differentiate filters from other paraphernalia such as cookers and water for injection. Both McCoy ea (1998) and Faran et al. (1998) found a strong association between the sharing of paraphernalia (cookers, water or filters) and HIV transmission. Vlahov et al. (1997) however, did differentiate paraphernalia and found that seropositive persons were more likely to have shared their cottons than seronegative people.

Crofts et al. (2000) detected the presence of hepatitis C virus on 40% of the used filters they examined. HCV can remain both present and viable in filters and will stay infectious for a longer period of time when the filter is protected against drying out (enrolled in aluminium foil) (Doerrbecker et al., 2013)

The correlation between filter sharing and HCV contamination has been confirmed by several epidemiological studies. Hagan et al. (2001) studied the sharing of filters and cookers (combined) and found an elevated risk for HCV seroconversion (adjusted relative risk of 5.9). Several cohort studies found cotton filter sharing to be a strong predictor for HCV seroconversion (adjusted relative risk of 16.4 – Bruandet et al., 2006; of 2.4-Thorpe et al., 2002 and of 2.83 – Hagan et al., 2010). Pouget et al. (2012), in a meta-analysis of 6 scientific publications on hepatitis C seroconversion in association to filter sharing (including the above articles) confirmed a relative risk of 2.61.

Filters and micro-organisms

As mentioned, most makeshift filters retain quite some liquid after injection. As most of them are manipulated with the hands or even with the mouth (people tend to tear the cigarette filter off of the cigarette using their teeth), these filters can be contaminated with bacteria or fungi, as shown by Scott (2008). Moist filters, if kept in a warm and protected environment, can even serve as a growth medium for these micro-organisms which are capable to infect people who reuse them. Such filters can thus induce several serious infections such as abscesses, ophthalmic candidiasis and endocarditis…

Tearing off the cigarette filter with the teeth has been found to be an independent risk factor for systemic candidiasis in people who inject drugs (Gambotti et al., 1996). In the same study, filter reuse seemed to be more prevalent amongst people who had systemic candidiasis (46%) than amongst controls (28%), though this was not statistically significant (p= 0.07).

The Sterifilt can be fitted to different types of syringes without manipulation of its membrane, thus lessening the risk of bacterial transmission from handling. However, this filter does not eliminate bacteria from the solution. Only sterilizing filters (0,22 micron) can remove most bacteria from solutions (Caflisch et al., 1997).

Cotton fever

"Cotton fever” is the name given to an inflammatory and pyrogenic response to, probably, the injection of bacteria (or parts of bacteria), endotoxins or fungi. These micro-organisms may be dead or alive when they induce this reaction. The symptoms that arise shortly after injection are headache, chills and rigors, dyspnoea, palpitations and fever (Harrison & Walls 1990). Usually, "cotton fever” lasts for several hours, and will then disappear. However, endocarditis and sepsis may induce the same symptoms; moreover, "cotton fever” has been found to be a precursor of systemic candidiasis (Gambotti et al. 1996); it might thus be that the microorganisms that can be responsible for this febrile reaction are capable of inducing a serious infection in time. People showing signs of cotton fever should thus be kept under observation.

Filtration and the injection of insoluble particles

Several complications arising from injecting drug use depend on the drugs injected as well as the characteristics of the filter used. Amongst these is the introduction of insoluble particles into the blood stream.

Insoluble particles may be introduced in illicit drugs through cutting agents (talc, starch) (Cole et al., 2010). However, they are often present in higher concentrations in oral pharmaceutical formulations which are not intended for intravenous administration (e.g. methadone, buprenorphine, methylphenidate, oxycodone - Hind, 1990; Rosenblum et al., 1997). Generally, these tablets are crushed or capsules opened, the resulting powder is mixed with water, and the suspension is injected.

Various complications, ranging from minor to severe, are associated with the intromission of these foreign bodies. At the site of injection, sterile abscesses, cellulites and ulcers can occur, which increase the risk of infection at these sites (Del Giudice, 2004; Hahn et al., 1969). After injection, insoluble particles such as talc and cellulose will stay intact and move along with the blood stream, blocking the first vessels too small to pass. Repeated administration can thus lead to severe pulmonary and cardiac complications such as pulmonary embolism, pulmonary failure, fibrosis, right heart failure and talcosis (Lamb and Roberts, 1972; Sieniewicz and Nidecker, 1980; Gorun et al., 2008; Marschke et al., 1975; Kringsholm and Christoffersen, 1987). Talcosis resembles the miners’ disease that is called silicosis. People suffering from talcosis will experience moderate to severe dyspnoea, can develop cyanosis and even die (Sieniewicz and Nidecker, 1980; Paré et al., 1998). This condition can take one to several years to develop, but once present, the symptoms are irreversible and continue to develop despite of discontinuation of drug use (Paré et al., 1998).

Poor filtration has been suggested to be one of the risk factors for the development of these complications (Jampol et al., 1981). Indeed, all filters used by PWID will eliminate some of these particles, but not with the same efficacy. The size of the majority of insoluble particles involved in the development of these complications is within the range of 9 μm to 23 μm (medium of 14 μm) (Abraham and Brambilla, 1980). Cigarette filters, commonly used by injecting drug users, eliminate less than half of all particles above 10 μm (Scott, 2002). Injecting drug use syringe filters (IDUSF) have been specifically conceived for drug use and are capable of eliminating the large majority of insoluble particles (Roux et al., 2011) These filters are not designed to sterilise a non-sterile solution, but to promote single use of filters and to eliminate particles of over 10 micron, in accordance with the European Pharmacopeia concerning injectable preparations.

The Sterifilt is one of those filters. It eliminates at least 93% of all particles above 10 microns and can thus substantially reduce the serious complications due to the injection of insoluble particles (Scott, 2008; Scott, 2002; Roux et al., 2011).

At the same time, the Sterifilt thus reduces the risk of particles clogging or blocking the needle. It also protects the tip of the needle from hitting the bottom of the spoon; the needle thus remains sharp and will cause less vein damage.


  • Provide individually packed, sterile filters adapted to the injection of illicit drugs and oral pharmaceutical formulations.
  • A new sterile filter should be used for every injection. Filters should thus be distributed in the quantity requested by PWID, without limitation on their number.
  • Discourage filter reuse. The distribution of filters that retain virtually no active compound can motivate PWID to use these (as they hardly lose any active compound) and discourage filter reuse.
  • Instead of reusing cottons, some people will accept to set aside some of their powder or a small piece of their pill to diminish withdrawal symptoms.
  • Encourage people to use the filter with the smallest pore size possible to prevent harms from insoluble particles.
  • Encourage clients to switch to drugs causing the least health problems possible. Sometimes, the same drug of a different brand contains less harmful tablet fillers. In that case, this drug should be privileged.
  • Inform people on the harms associated with not using filters, the use of non-sterile filters, the manipulation of filters and the reuse or sharing of filters.
  • Provide also other harm reduction tools such as syringes, cookers, acids, sterile water for injection, alcohol pads…


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