Stable Isotope Proﬁling of
Internet Sourced Viagra® and
generic- Viagra Tablets
H.F.Kemp1 and W.Meier-Augenstein1,2
1Stable Isotope Forensic Lab, James Huon Instute, Dundee, DD2 5DA, UK
2Environmental & Forensic Science Group, Robert Gordon University, Aberdeen AB10 1FR, UK
Viagra® manufactured by Pﬁzer was the
ﬁrst prescripon drug for the treatment of
erecle dysfuncon (ED), a condion that is
esmated to aﬀect 1 in 10 men at some
stage in their lives (1). Viagra® contains the
acve pharmaceucal ingredient (API)
sildenaﬁl, as the citrate salt.
Since its ﬁrst producon in 1998, Viagra®
has generated well in excess of $10 billion
US dollars in sales (2) and with Pﬁzers’
patent extended to April 2020 (3) it sll
remains the only sildenaﬁl-based treatment
opon for suﬀerers of ED in the US. There
are no legal generic-Viagra formulaons
available in the US. However, formulaons
containing sildenaﬁl citrate as API are
widely available over the internet and oen
sold as ‘generic Viagra’. These cheaper
alternaves are oen manufactured under
less than ideal condions with lile or no
QA/QC procedures in place.
The World Health Organisaon recognised
the scale of the problem in its 2010 bullen
“Growing threat from counterfeit
medicines” (4) and quotes a Dutch study
cited in the Internaonal Journal of Clinical
Pracce in which from a cohort of 370
seized Viagra samples, only 10 were
genuine. In a more recent study (5),
Campbell et al., analysed 22 samples of
Viagra sourced from the internet and found
only 4 to be authenc, based on the %API.
During September 2012, we sourced a
variety of tablets sold for the treatment of
ED which claimed to have sildenaﬁl citrate
as API. Viagra®, generic-Viagra, Kamagra,
Silagra and Filagra tablets were ordered via
the internet and supplied from both
UK/European-based pharmacies as well as
overseas suppliers (Hong Kong, India,
In this small-scale pilot study, we present
results from bulk 2H/18O and 13C/15N stable
isotope analysis performed on crushed
tablets from 23 samples of
internet-sourced tablets sold for the
treatment of ED and purported to contain
sildenaﬁl citrate as API.
1. www.healthcare.org.uk; 2. www.moneynews.com; 3. US Patent &
trademark oﬃce (www.uspto.gov); 4. WHO bullen 2010;88:247-248; 5.
Campbell et al., Viagra (sildenaﬁl citrate) ordered via the internet is
rarely genuine. J.Sex Med. Aug 2012 9, (S4), p248; 6. NicDaéid et al.,
Using isotopic fraconaon to link precursor to product in the synthesis
of (±)-mephedrone. A new tool for combang ‘legal high’ drugs; Anal.
Chem. 2012 84, 8691-8696; DOI: 10.1021/ac3019069.
• The technique of mul-variant stable isotope analysis shows the potenal to be able to
disnguish between diﬀerent tablets sold as Pﬁzer-branded Viagra® and generic Viagra.
• Further work is required in the ﬁrst instance to a) authencate samples sold as branded Pﬁzer
Viagra®, b) fully chemically characterise the 23 tablets analysed for their stable isotope proﬁle in
this study and c) expand the database.
Materials & Methods
Figure 4: Principal
Component Plot with
ﬁrst and second
48% and 39%,
respecvely of all
variability in the data.
Samples believed to be
products are circled in
Table 1: List of products ordered via the internet during September 2012. All products claimed to have sildenaﬁl
citrate as acve pharmaceucal ingredient (API) and were sold for the treatment of erecle dysfuncon.
One tablet from each foil-sealed packet was selected and any outer tablet-coang removed using a scalpel blade (Figure 2).
Each tablet was ground to a ﬁne powder using an agate pestle & mortar. Powdered tablets were placed into glass vials
(Figure 3) and stored under vacuum in sealed desiccators over phosphorus pentoxide before being weighed into either
silver capsules or n capsules prior to 2H/18O, 13C/15N analysis respecvely.
Bulk 2H, 18O, 15N and 13C isotope analysis
Stable isotope analysis of powdered tablet samples by Connuous Flow – Isotope Rao Mass Spectrometry (CF-IRMS) was
carried out as described elsewhere (NicDaéid et al., 2012).
• Principal Component Analysis (PCA) of data obtained from bulk 2H, 18O, 13C and 15N stable isotope analysis of all powdered
tablets splits the sample cohort into 5 disnct clusters (Figure 4).
• PCA shows products V1-V6-WP form two disnct groups, V1-V5 in one group and V6-WP alone in a second group. All six
products were sold as branded Pﬁzer Viagra®. Products V1-V5 were ordered from UK/European-based online pharmacies
and required the compleon of an online pro-forma consultaon. Samples V1-V5 were supplied in Pﬁzer-branded sealed
boxes complete with paent informaon leaﬂets. Product V6-WP was supplied from a PO Box in Hong Kong and arrived
as a single foil sachet in a jiﬀy bag without any documentaon. In addion, diﬀerences were noted with regards to
posioning of batch No. and expiry date on the reverse of the blister packs between product V6-WP and products V1-V5
(whose blister packs were all of consistent appearance).
• No signiﬁcant diﬀerence was observed between branded Pﬁzer Viagra® samples V1-V5 believed to be authenc (see
above) despite diﬀerent API content (V1-V4: 50mg API and V5: 100mg API).
• The ght clustering of the 9 Kamagra tablets K1 and K2A-H, the 5 “authenc” Pﬁzer products V1-V5, and the 4 Filagraxxx
products F2-F5 indicated a relavely low degree of inter- and intra-batch variability within these 3 diﬀerent products.
Product sold as Batch No.
V3 Pfizer Viagra A246105U blue diamond 50mg
V5 Pfizer Viagra B101483652 blue diamond 100mg
S1-WP Silagra-100 DP1144 lilac teardrop 100mg
Filagraxxx-100 'fruit chew'
Filagraxxx-100 'fruit chew'
F4-LY Filagraxxx-100 'fruit chew' F-144001 light yellow diamond 100mg
Filagraxxx-100 'fruit chew'
C1-WP Cenforce-50 T-02501L11 light blue diamond 50mg
C2-BP Cenforce-Professional T-09502D12 light blue diamond 100mg (sublingual)
Tablet Summary (see Table 1)
16 products were ordered. With the excepon of
our product reference K2, 1 tablet per product
was analysed. Product K2 was supplied as 8
sachets of 4 tablets; 1 tablet from each of the 8
foil-sachets were analysed (K2A-H). Tablets
V1-V5, S1 & K1 were supplied in proprietary
packaging (sealed boxes; Figure 1) with paent
informaon leaﬂets whereas the remaining were
received simply as a foil sachet in jiﬀy
bags/cardboard envelopes (V6-WP, C1-C2, F1-F5,
K2) with lile or no documentaon.
Figure 1 Figure 2 Figure 3