Tagamet a.k.a. cimetidine is an immunomodulator and potentially useful
at least for those people whose T-helper/T-suppressor ratio is too low
due to histamine induced T-suppressor cells. Here some information and
references about immunomodulatory action of cimetidine:

Evaluation of Histamine2-Receptor-Antagonists (H2-Antagonists)
http://dacc.bsd.uchicago.edu/drug/Bulletins/n1096.html

   "H2-antagonists have been found to improve the function of various
   parts of the immune system.(6,7) There have been anecdotal reports
   of the usefulness of histamine-antagonists in the treatment of
   various cancers refractory to standard treatments (eg, lung
   cancer, malignant melanoma). The proposed mechanism of
   immunomodulative effects is the inhibition of suppressor T-
   lymphocyte activity, an increase in interleukin-2 production, and
   an enhancement of natural killer cell activity. Hahm and
   colleagues compared the in vitro effects of cimetidine, ranitidine
   and famotidine on peripheral blood mononuclear cells in normal
   controls and patients with gastric cancer. They concluded that
   cimetidine had the strongest and famotidine had the weakest
   immunomodulating effect. Cimetidine was found to proliferate
   peripheral blood mononuclear cells and increase cytotoxic
   capability. Famotidine had no effects on lymphoproliferation and
   cytotoxicity. Ranitidine and famotidine were also inferior to
   cimetidine in inhibiting suppressor T-cell activity, increasing
   interleukin-2 production and enhancing natural killer cell
   activity. The authors speculated that this difference may be due
   to the fact that famotidine and ranitidine lack the imidazole
   nucleus common to histamine and cimetidine."

Griswold DE, Alessi S, Badger AM, Poste G, Hanna N.
Inhibition of T suppressor cell expression by histamine type 2 (H2)
receptor antagonists.
J Immunol. 1984 Jun;132(6):3054-7.
PMID: 6202771 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=6
202771&dopt=Abstract

Nanda NK, Nath I.
Characteristics of histamine receptors present on suppressor T cells
in "healthy individuals".
Int J Immunopharmacol. 1985;7(4):587-95.
PMID: 2931386 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
931386&dopt=Abstract

Griswold DE, Alessi S, Badger AM, Poste G, Hanna N.
Differential sensitivity of T suppressor cell expression to inhibition
by histamine type 2 receptor antagonists.
J Immunol. 1986 Sep 15;137(6):1811-5.
PMID: 2875110 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
875110&dopt=Abstract

Kumar A, Cleveland RP.
"Immunoregulatory effects of cimetidine: inhibition of suppressor cell
effector function in vivo".
Immunopharmacol Immunotoxicol. 1988;10(3):327-32.
PMID: 2974050 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
974050&dopt=Abstract

Kumar A.
Cimetidine: an immunomodulator.
DICP. 1990 Mar;24(3):289-95. Review.
PMID: 2138376 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
138376&dopt=Abstract

Brockmeyer NH, Kreuzfelder E, Bluhm C, Shen G, Scheiermann E, Keinecke
HO, Ohnhaus EE.
Immunomodulation of cimetidine in healthy volunteers.
Klin Wochenschr. 1989 Jan 4;67(1):26-30.
PMID: 2522158 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
522158&dopt=Abstract

Gifford RR, Voss BV, Schmidtke JR, Ferguson RM.
Histamine type-2 receptor antagonist immune modulation. I. Increased
cell-mediated cytotoxicity in normal and in down-regulated systems.
Surgery. 1988 Feb;103(2):184-92.
PMID: 2963399 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
963399&dopt=Abstract

Brockmeyer NH, Kreuzfelder E, Guttmann W, Mertins L, Goos M, Ohnhaus
EE.
Cimetidine and the immuno-response in healthy volunteers.
J Invest Dermatol. 1989 Dec;93(6):757-61.
PMID: 2573637 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
573637&dopt=Abstract

Richtsmeier WJ, Eisele D.
In vivo anergy reversal with cimetidine in patients with cancer.
Arch Otolaryngol Head Neck Surg. 1986 Oct;112(10):1074-7.
PMID: 3755977 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3
755977&dopt=Abstract

Flodgren P, Sjogren HO.
Influence in vitro on NK and K cell activities by cimetidine and
indomethacin with and without simultaneous exposure to interferon.
Cancer Immunol Immunother. 1985;19(1):28-34.
PMID: 3844972 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3
844972&dopt=Abstract

Cimetidine may be useful in some type of cancers, for example in
certain type of colorectals cancers. In the study below the 64
colorectal cancer patients were treated with surgery
and5-fluorouracil. One group additionally received 800 mg/day of
cimetidine.  The 10-year survival rate was about 85% in the cimetidine
group and about 50% un the control group. The difference was
statistically significant.

Matsumoto S, Imaeda Y, Umemoto S, Kobayashi K, Suzuki H, Okamoto T.  
Cimetidine increases survival of colorectal cancer patients with high
levels of
sialyl Lewis-X and sialyl Lewis-A epitope expression on tumour cells.
Br J Cancer. 2002 Jan 21;86(2):161-7.
PMID: 11870500 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1
1870500&dopt=Abstract

Abstract:
       
  "Cimetidine has been shown to have beneficial effects in colorectal
   cancer patients. In this study, a total of 64 colorectal cancer
   patients who received curative operation were examined for the
   effects of cimetidine treatment on survival and recurrence. The
   cimetidine group was given 800 mg day-1 of cimetidine orally
   together with 200 mg day-1 of 5-fluorouracil, while the control
   group received 5-fluorouracil alone. The treatment was initiated 2
   weeks after the operation and terminated after 1 year. Robust
   beneficial effects of cimetidine were noted: the 10-year survival
   rate of the cimetidine group was 84.6% whereas that of control
   group was 49.8% (P<0.0001). According to our previous observations
   that cimetidine blocked the expression of E-selectin on vascular
   endothelium and inhibited the adhesion of cancer cells to the
   endothelium, we have further stratified the patients according to
   the expression levels of sialyl Lewis antigens X (sLx) and A
   (sLa). We found that cimetidine treatment was particularly
   effective in patients whose tumour had higher sLx and sLa antigen
   levels. For example, the 10-year cumulative survival rate of the
   cimetidine group with higher CSLEX staining, recognizing sLx, of
   tumours was 95.5%, whereas that of control group was 35.1% (P=
   0.0001). In contrast, in the group of patients with no or low
   levels CSLEX staining, cimetidine did not show significant
   beneficial effect (the 10-year survival rate of the cimetidine
   group was 70.0% and that of control group was 85.7% (P=n.s.)).
   These results clearly indicate that cimetidine treatment
   dramatically improved survival in colorectal cancer patients with
   tumour cells expressing high levels of sLx and sLa."

Other references about cimetidine and cancer:

Hayashi A, Kobayashi K, Imaeda Y, Matsumoto S.
[Cimetidine inhibits the adhesion of cancer cells with sialyl Lewis
epitope onto the vascular endothelium]
Gan To Kagaku Ryoho. 2003 Oct;30(11):1788-90. Japanese.
PMID: 14619520 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstra
ctPlus&list_uids=14619520

Yoshimatsu K, Ishibashi K, Hashimoto M, Umehara A, Yokomizo H, Yoshida
K, Fujimoto T, Iwasaki K, Ogawa K.
[Effect of cimetidine with chemotherapy on stage IV colorectal cancer]
Gan To Kagaku Ryoho. 2003 Oct;30(11):1794-7. Japanese.
PMID: 14619522 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstra
ctPlus&list_uids=14619522

Kobayashi K, Matsumoto S, Morishima T, Kawabe T, Okamoto T. Cimetidine
inhibits cancer cell adhesion to endothelial cells and prevents
metastasis by blocking E-selectin expression.
Cancer Res. 2000 Jul 15;60(14):3978-84.
PMID: 10919677 [PubMed - indexed for MEDLINE]
<http://cancerres.aacrjournals.org/cgi/content/full/60/14/3978>

Siegers CP, Andresen S, Keogh JP.
Does cimetidine improve prospects for cancer patients?. A reappraisal
of the evidence to date.
Digestion. 1999 Sep-Oct;60(5):415-21. Review.
PMID: 10473965 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1
0473965&dopt=Abstract

There has been also some doubts that very long term use of cimetidine
could be associated with increased risk of some type of cancers.

Cimetedine's patent has already expired. Unfortunately this is bound
to prevent or severely restrict investments into its further research
and testing by the private sector. We just have to hope that the
public sector picks up the research.